The word dawa (pl. du'at) is derived from du'a means to call, invite or summon, and thus the term dai denotes, "he who summons", whose corresponding term in English is "missionary" (derived from the Latin, mittere). The word dawa is also used in the sense of prayers, such as dawat al-mazlum (prayer of the oppressed), or dawa bi'l shifa (prayer of the health). The word dawat virtually originated in the time of Imam Jafar Sadik, and Abdullah bin Maymun had founded the Ismaili dawa organisation in Basra.
T.W. Arnold writes in "The Preaching of Islam" (Aligarh, 1896, p. 277) that, "The Ismailis were the master of organisation and tactics at the time of Abdullah bin Maymun." W. Ivanow writes in "Collectanea" (Holland, 1948, p. 20) that, "The only branch of Islam in which the preaching of religion, dawat, was not only organised but even considered of special importance, was Ismailism." According to "The Encyclopaedia of Islam" (Leiden, 1965, 2nd vol., p. 168), "The word dawat is well known as applied to the wide-spread Ismaili propaganda movement, appealing to Muslims to give their allegiance to an Imam descended from Ismail bin Jafar Sadik."
Soon afterwards, Salamia became the headquarters of Ismaili dawat after Basra, while Yamen later on became the dai generating hub. Indeed, very little is known about the actual mission (dawa) system of early Ismailism, but it is however certain that the Ismaili mission was brisk and pervasive throughout the Islamic regions. In the broadest terms, it seems that Muhammad bin Ismail was represented by twelve hujjats in different regions, and beneath the hujjats, a hierarchy of missionaries (dais) conducted the different tasks of initiation and instruction. The Ismaili dais stimulated a network of the mission in many parts of the Abbasid empire and there was plenty of its activity even outside it. They fully exploited the socio-economic conditions of the weaker sections of society to attract them towards the mission on one hand, and the philosophical interpretations of the teachings of Islam to attract the thinking sections of the society on the other.
For purposes of mission, the world was divided into twelve parts, each being called jazira (usually translated as an island), known as the island of the earth (jazira al-arad). It is difficult to say whether jazira really meant an island. One can broadly agree with W.Ivanow when he says: "It appears that in this sense jazira does not mean the island, as it usually means, but is taken here in its basic sense, from the root j-z-r = to cut off, and therefore means a slice, cutting, or a part, a section. Therefore the expression 12 jazair should be translated as the 12 sections of the world population. They are: Arabs, Turks, Berbars, Negroes, Abyssinians, Khazras, China, Daylam, Rum and Saqaliba. Thus this classification is partly based on geographical, and partly on ethnographical principle, and plainly belongs to the fourth/tenth century." (vide "The Rise of the Fatimids", Calcutta, 1942, p. 21)