The origin of the cardinalate cannot be traced back to a single event. Instead, it is a slow and laborious process that started in the very early centuries of the Roman Church. Its origin blurs with that of the Roman presbyterium. The presbyterium evolved through the centuries and within it the most important members of the group, those who head the titular churches (the oldest ones in the city of Rome), acquired gradually a more prominent role. Those were the pre-cardinals. After many centuries, the office of cardinal continues to be modified and redefined as it is after the papacy the most important component of a living community that exists in our midst: the Roman Catholic Church. The peseudo-cardinals are those who were named by an anti-pope; and the quasi-cardinals are the ones that a pope intended to promote to the cardinalate and for different reasons the elevation did not take place.
The term "cardinal" was initially used as an adjective--"cardinati" or "incardinati"--that qualified the priests of the titular churches that were assigned to help in the liturgical functions of one of the four major basilicas (St. Peter, St. Paul, St. Mary and St. Lawrence). The service in St. John Lateran was reserved for the bishop cardinals of the suburbicarian sees. The term "cardo" meant a beam used to fortify a structure. Thus, these priests were referred to as presbyter cardinals. In the course of the centuries, the liturgical functions gave way to administrative and gubernatorial ones. Another meaning given to the term "cardo" is hinge. The pope is the hinge of the door of the church, which opens or closes them. In this respect, because of the close colaboration between the titular priests of the city of Rome and its bishop, they were called cardinals.
At the same time that this change in the functions of the presbyterium was occurring, the church was struggling to free itself from the intervention of external forces that meddled in the election of its head, the bishop of Rome, the Roman Pontiff. Whether it was against the Roman emperor, the Ostrogothic king of Italy, the Byzantine emperor, the Carolingian monarch, or the Holy Roman German emperor, the Roman church sustained a multi-secular struggle for the establishment of an electoral process for the selection of the pope that would be controlled only by its clergy.
The fact that the bishop-cardinals, the priest-cardinals and the deacon-cardinals eventually became the exclusive electors gave this group of ecclesiastics a highly relevant position in the hierarchy of the Roman Church.
The development of the Roman presbyterium and the attempts of the Church to free the electoral process of its ruler from lay hands are like two lines whose courses approach closer with time until they intersect for the first time in 769 with the electoral decree of Stephen IV. This document established that the pope was to be elected by and from among the deacons and priests of the Roman Church. Within a short period of time, the two lines separate again for 290 years until 1059 when Nicholas II issues the decree In Nomine Domini by which the cardinal bishops became the sole electors of the Roman Pontiff. Later, in 1179, the rest of the cardinals (priests and deacons) joined the bishops in the electoral process as established by the decree Licet de vitanda of Pope Alexander III in the III Lateran Council. These two documents recognized and confirmed the leading role of the cardinals in the Church. Their role had been evolving and consolidating through the centuries. From 1179 on, the cardinals became second only to the pope in the church hierarchy, and received many new responsibilities, honors and privileges.http://www2.fiu.edu/~mirandas/preface.htm