Answer:
RCIA stands for Rite of Catholic Initiation for Adults. The classes are for anyone who is interested or has questions about the Catholic Church and what she teaches, whether they are thinking of joining or not. RCIA is a process and there are five different stages: Inquiry, Catechumenate, Purification and Preparation, Initiation, and Mystagogy.
In the first stage, Inquiry, you are simply discovering what the Catholic Church teaches and finding answers to your questions. There is no need for commitment of any kind. "Your main task here is to explore and develop your faith enough so you can make an informed initial decision about entering the Catholic Church" (link added).
Once you enter the Catechumenate stage you have decided that you want to join the Catholic Church. In this stage you are called a catechumen, and go through catechesis, which is "the use of written and spoken words [...] to pass down the Gospel of Jesus Christ" (Catholic Dictionary, 167). Catechumens learn the Catechism and more about the Catholic faith.
The Purification and Preparation stage is the last stage before you are initiated into the Catholic Church at Easter. It takes place during the Lent before you are received into the Church. It is an intense period that calls for reflection and deeper conversion. You also go through Scriptural meditation, focusing on the Gospels, to prepare you to commit your life to Christ.
Finally comes the Easter Vigil and your initiation day! During the Easter Vigil Mass, you will receive the Sacraments of Confirmation, Eucharist, and Baptism (if you haven't been baptized already). "Initiation itself [is] the culmination of the whole process" (link added).
After you are received into the Church, you go through the Mystagogy stage. In this stage you learn about what the Church teaches again. Since you have now received the graces from the Sacraments of Initiation, you can now understand the mysteries better and see them through eyes of faith. It also helps you to stay strong in your new faith, so that you are not just shoved into the "real world" without any support.