What is the purpose of a defense to a criminal charge?

I want you to read the documents I upload and answer these questions in order please write the question then answer under it
1. What is the purpose of a defense to a criminal charge?
2. What is an affirmative defense?
3. What is the difference between justification and excuses? Give examples of justifications that might serve as defenses.
4. What fundamental claim is raised be the defense?