Once students complete the Sunday School program, they graduate to the Catechism classes. These classes prepare students for church membership, but more importantly, lead students into a closer and deeper relationship with God. The Heidelberg Catechism helps students discover just how powerful the gospel really is, and "how wide and long and high and deep is the love of Christ" (Eph 3:18).
If you trace back to the origin of the word Catechism, it comes from a Greek word (katecheo) which simply means to teach or to instruct. A catechism, then, is a teaching tool which is especially suited for oral instruction.  This is why the Heidelberg Catechism is set up in a question and answer format.
In addition, the answers in the Catechism are generally quite short and carefully structured.  All of this helps to make the Catechism a memorable and useful tool for instructing people in the basic truths of salvation in Jesus Christ. 
First published in 1563, the Heidelberg Catechism has been used by Christians around the world ever since.  The Catechism summarizes the major teachings of Holy Scripture in 129 memorable questions and answers, divided over 52 sections called Lord's Days. 
The first Lord's Day is on the theme of Comfort answerting the question "What is my only comfort in life and in death?" After the first Lord's Day, the rest of the questions are divided into three parts:
  1. Our sin & misery
  2. Our deliverance from sin
  3. Our thankfulness to God for such deliverance
It should be noted that the Catechism is not a replacement for Scripture.  It is meant to lead you deeper into the Word of God, not draw you away from it.  That's why under each Lord's Day you'll see lists of Bible references.  Even though each list is selective, it gives you an idea which Bible passages the Catechism is summarizing in each answer.