Honestly, I don't think much about sin. Looking at the dictionary, this is probably the closest definition of sin that I see: "an act regarded as a serious or regrettable fault, offense, or omission."
I have a hard time seeing it as a transgression of a divine law, as many definitions suggest. But I do believe that all people sin, and that sin should be avoided as much as possible. I don't think that a person who sins will be condemned though. This is partially because I do not believe in the idea of hell either.
At the same time though, I would like to believe that people who commit very grave sins, will be punished for those actions, but what exactly happens, I really have no opinion.
As with salvation, I must start off with saying that I do not think that Jesus died for our sins. I do not even believe that he necessarily was resurrected in a physical sense. To be honest, I think it is most likely that his body was left in a shallow grave, and probably was devoured by wild dogs and scavenging birds. However, I do believe that there was something. I like to think that there was a spiritual resurrection, but I wouldn't say for definite. All of that said, I do see that Jesus does signify something special.
I see the life of Jesus, death, and whatever happened afterwards, signifying something special. I don't think it was necessary, but a reminder of the love that God has for humankind (and I don't even see the actions of Jesus to be necessarily this symbol, but the story that arose from this, and the religion that arose from this). It represented the salvation that was already in place. It served as a reminder of what God already had in store, that people did not have to fear death, that we are saved, and that we didn't have to worry about being perfect.