In the Roman Catholic Church, what exactly is a sin?

Whenever I go to confession there are always some sins that I write down but am not absolutely sure if they are sins. Likewise there are other things I don't confess because I don't know if it would be a sin. I know that the definition of a sin is any act that is don without faith or disregarding God, but how do I know if defending myself or fighting with a friend because they have done something wrong is a sin? Can you give me more examples apart from lying, steeling and physically hurting another person? I'm really confused.