The RCIA stands for the Rite of Christian Initiation of Adults
RCIA in its purest form is for those adults who have not yet been baptised-however in reality those that are converting from other Christian religions often go through the RCIA as well- and certain provisions need to be made for this.
The RCIA is a process by which adults are introduced to the life of the Catholic Church through catechesis and liturgy.
When an adult becomes Catholic they receive all 3 sacraments of initiation at the same time-Baptism, Eucharist and Confirmation.
Vatican II said that RCIA should be the proper way of initiating adults.