Sin
In the Old Testament God gave "The Law" to the Jews. The most famous part of the Law was the Ten Commandments. It was a list of things the Jews weren't allowed to do. So for Jews defining what was a sin came down to what was allowed or not allowed in the Law.
But Jesus fulfilled the Law and it was nailed to the cross with him. So Christians are not under the Law. We don't have a list of things we can and can't do. For us, sin is defined by motive. Not what you do, but why you do it. We should always put God first. Our actions should always be motivated by love for others. Sin is basically selfishness. It's when I think I'm more important than others, or more important than God.
Two Christians could do the same action, and for one it may be a sin and for the other not. Even if I am giving money to the poor, but because I am doing it so everyone sees me and thinks I am a great guy. That is selfish, that's pride, it's all about me. It's a sin.
Some sins are worse than others. But surprisingly the worst ones aren't murder and rape, they are pride and greed.
And finally, one important thing about sin. We all sin. God will confront our sins one by one as he helps us to change our behaviour over the years. But he decides which order. So, don't judge other Christians for their sin. God will deal with them in his time. You just worry about your own sin. Besides, sin is about motive and you do not know what is going on inside the other person.
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