The title Consul is used for the official representatives of the government of one state in the territory of another, normally acting to assist and protect the citizens of the consul's own country, and to facilitate trade and friendship between the people of the country to whom he or she is accredited and the country of which he or she is a representative. This distinguishes the consul from the ambassador or chargé d'affaires, who is, technically, a representative from one head of state to another. Thus, while there is but one ambassador representing a nation's head of state to another, and his or her duties revolve around diplomatic relations between the two countries, there may be several consuls, one in each of several main cities, providing assistance with bureaucratic issues to both the citizens of the consul's own country travelling or living abroad, and to the citizens of the country the consul is in who wish to travel to or trade with the consul's country.

Antecedent: The Classical Greek Proxenos
Consuls were originally the highest magistrates of the Roman Republic. The term was revived by the city-state of Genoa which - unlike Rome - bestowed it on various state officials, not necessarily restricted to the highest. Among these were Genoese officials stationed in various Mediterranean ports, whose role included duties similar to those of the modern Consul, i.e. helping Genoese merchants and sailors in difficulties with the local authorities. This institution, with its name, was eventually emulated by other powers.

Historical development of the term
The office of a Consul is termed a Consulate, and is usually subordinate to the state's main representation in that foreign country, nowadays usually an Embassy or High Commission usually in the capital city of the host nation. In the capital, the consulate may be a part of the embassy itself. A consul of higher rank is termed a "consul general", and his or her office a "consulate general." He or she typically has several Consuls and Vice-Consuls working under him/her. Consulates-general need not be in the capital city, but instead in the most appropriate cities. In the United States, for example, many countries have a consulate-general in New York City, and some have consulates-general in several cities (e.g. Chicago, Houston, Los Angeles, San Francisco).
Consuls of various ranks may have specific legal authority for certain activities, such as notarizing documents. As such, diplomatic personnel with other responsibilities may receive consular commissions. Aside from those outlined in the Vienna conventions, there are few formal requirements outlining what a consular official must do. For example, for some countries, consular officials may be responsible for the issuance of visas; other countries may limit "consular services" to providing assistance to compatriots, legalization of documents, etc. Nonetheless, consulates proper will be headed by consuls of various ranks, even if such officials have little or no connection with the more limited sense of consular service.
Contrary to popular belief, although many of the staff of consulates may be career diplomats they do not generally have diplomatic immunity (unless they are also accredited as such). Immunities and privileges for consuls and accredited staff of consulates under the relevant international conventions are generally limited to actions undertaken in their official capacity and, with respect to the consulate itself, to those required for official duties. In practice, the extension and application of consular privileges and immunities can be subject to wide discrepancies from country to country.
Consulates are more numerous than diplomatic missions (e.g. embassies), since the latter are posted only in a foreign nation's capital (exceptionally even outside the country, in case of a multiple mandate, e.g. a minor power may well accredit a single Ambassador with several neighbouring states of modest relative importance that are not considered important allies), while consular ones are also posted in various cities throughout the country, especially centres of economic activity, or wherever there is a significant population of its citizens (expatriates) in residence.
Consulates are subordinate posts of their home country's diplomatic mission (usually an embassy, in the capital city of the host country). Diplomatic missions are established in international law under the Vienna Convention on Diplomatic Relations, while consulates-general and consulates are established in international law under the Vienna Convention on Consular Relations (see external link below). Formally, at least within the US system, the consular career (ranking in descending order: Consul-General, Consul, Vice-Consul, Honorary Consul) forms a different hierarchy from the diplomats in the strict sense. However, it is common for individuals to be transferred from one hierarchy to the other, and for consular officials to serve in a capital carrying out strictly consular duties within the 'consular section' of a diplomatic post, e.g. within an embassy.
Activities of a consulate include protecting the interests of their citizens temporarily or permanently resident in the host country, issuing passports; issuing visas to foreigners and public diplomacy. However, the principal role of a consulate lies traditionally in promoting trade - assisting companies to invest and to import and export goods and services both inwardly to their home country and outward to their host country. And although it is never admitted publicly, consulates, like embassies, may also gather intelligence information from the assigned country. This is especially important if the consulate is located in a port city.
Between Commonwealth countries, both diplomatic and consular activities may be undertaken by a High Commission in the capital, although larger Commonwealth nations generally also have consulates and consulates-general in major cities. For example, Toronto in Canada, Sydney in Australia and Auckland, New Zealand, are of greater economic importance than their respective national capitals, hence the need for consulates there. In British colonies, most notably Hong Kong before its transfer to China in 1997, senior envoys in these missions were usually known as Commissioners, but are now known as Consuls-General, subordinate to an Embassy in Beijng.
Some consuls are not career officials of the represented state at all; some are locally-engaged staff with the nationality of the sending country (see Chapter 1, Section 1, Article 22 of convention), and in smaller cities, or in cities that are very distant from full-time diplomatic missions, a foreign government which feels that some form of representation is nevertheless desirable may appoint a person who has not hitherto been part of their diplomatic service to fulfill this role. In some instances, the honorary consul may not be a citizen of the sending country, and may well combine the job with their own (often commercial) private activities, in which case they are usually given the title of honorary consul. Many members of the public are not aware that honorary consuls are not full-time diplomats. Graham Greene used this position as the title of his 1973 novel The Honorary Consul.

Colonial and similar roles
When a state falls under the "amical" protection of a stronger (often colonial) power, the latter is usually represented by a high ranking diplomatic and/or gubernatorial officer, such as a Resident general, Resident Minister or High Commissioner. However, if there is no such representation (in modern terms often at ambassadorial level), the task may fall to the only available 'diplomatic' alternative: consular representation.
This arrangement lasted until 1 March 1900, when the archipelago was annexed by imperial Germany, with the exception of the eastern islands, which remained under U.S. control and became the territory of American Samoa).

in the German West African Kamerun, 6 July 1884 - 26 June 1885, provisional consul Heinrich Randad filled the void between the first Reichskommissar (titled - for West Africa, 5-6 July 1884 only) and the subsequent series of regular incumbents
In parts of present Nigeria, British Consuls were in charge of the following West African protectorates:

• the Bight of Benin May 1852 - 6 August 1861
  the Bight of Biafra 30 June 1849 - 6 August 1861
  the Bights of Biafra and Benin since the merger of the two above on 6 August 1861; the last incumbent was promoted on 5 June 1885 to stay on as first Consul general (of two) of the Bights
  From 7 November 1889, Samoa, previously a Polynesian kingdom, was governed by the joint German-British-U.S. Samoa Tripartite Convention, which made Samoa a protectorate of those three powers. On 10 June 1899, a provisional (colonial) government sui generis was formed, consisting of the consuls of the three protecting powers:
  
  
  
  • Friedrich Rose (German Consul) (b. 1855 - d. 1922)
    Ernest George Berkeley Maxse (British Consul) (b. 1863 - d. 1943) - to 23 June 1899, succeeded by a mister Nair (acting British consul)
    Luther Wood Osborn (U.S. Consul) (b. 1843 - d. 1901).
    On Tonga, a British protectorate since 1900, the British Empire was only represented by its consuls from 1901 until Tongan independence in 1970. From 1901 until 1952, the protectorate was also under the administrative authority of the High Commissioner of the British Western Pacific Territories (always the British Governor of Fiji). Protectorates
    Even within another state, a foreign power often has extraterritorial rights over its official representation (such as a consulate). If such concessions are obtained, they are often justified as protection of the foreign religion (especially in the case of Christians in a Muslim state, such as in the so-called capitulations by the Ottoman Sultan to -Christian- European powers) and extend to the foreign power's expatriate citizens. In some cases, the foreign power actually took control of certain aspects of the local administration in order to see to the rights of expatriate citizens.
    A few examples:
    In other cases a part of a weaker state is complete handed over (without the formal surrender of 'naked' sovereignty) to be administered as a concession, including the indigenous local population:
    
    In 1261, the Genoese, having assisted Byzantine Emperor Michael VIII Palæologus in reconquering Constantinople, were rewarded with Smyrna and Pera as well as a Black Sea trade monopoly. They rapidly developed markets along the Black Sea's shores, the principal one being Caffa, and carried on a brisk trade, exporting mainly wine, oil, woollens and silks, and importing skins, furs, corn and Persian goods. A consulate general of the empire of Gazaria was established as the local government of these colonies.
    In 1855, Sir John Bowring signed a new treaty whereby Siam agreed to the appointment of a British consul in Bangkok and to that official exercising full extraterritorial powers. British subjects were permitted to own land in certain defined districts, customs and port dues and land revenues were fixed, and many new trade facilities were granted. This important arrangement was followed at intervals by similar treaties with the other powers, the last two being those with Japan in 1898 and Russia in 1899. A later convention established a second British consular district in northern Siam, while Britain and France both appointed vice-consuls in different parts of the country. Thus Westerners in Siam (the Chinese had no consul) could only be tried for criminal offences, or sued in civil cases, in their own consular courts. A large portion of the work of the foreign Consuls, especially the British, was consequently judicial and in 1901 the British government appointed a special judge and an assistant judge to this post. Meanwhile, trade steadily increased, especially with Great Britain and the British neighbouring colonies of Hong Kong and Singapore.
    In the small Italian concession territory in Tientsin (a treaty port, now Tianjin), the Consul was in charge of the entire local administration.
    A long list of French consuls-general in Shanghai served as both overseers of the French concession in this Chinese port as well as presidents of the metropolis's Municipal Council. This arrangement lasted from January 1848 until 15 May 1946 (shortly after the 28 February formal restoration of power from France to China). Occupied territories under similar control
    This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
    
    CONSUL.CC - The First Worldwide Consul Directory (non-profit-organization)
    WorldStatesmen- follow the links here (per colonizing power) or see under each present country