A catechism is a series of questions and answers. Catholic children study a catechism as part of their religious education; it outlines the principal tenets of their faith.
If you're looking to understand the Christian faith, the catechism is a good place to begin. It outlines people's relationship to God, man's need for God, and the importance of the scriptures and the principles of the faith. It is meant to be digested over time, though children used to commit portions of it to memory.