What is a sin?

For example, if you find yourself obsessing on someone, on some physical injury, or on some painful conversation, you risk becoming a bitter person obsessed with revenge, which is the sin of anger. If you let your soul feel total contempt and loathing for someone, and it you're always insulting and censuring them in your mind, you're living with envy. If you give in to a feeling of malaise and exhaustion, that's laziness. On the other hand, if you find yourself entertaining ideas so pleasant that you could rest in them forever, only yo realize that these thoughts are all about your own natural goodness, accomplishments, intelligence, talents, position, or beauty, this is pride. And if you dwell on your wealth and what you own (or want to own), then that's greed. If you're preoccupied with lots and lots of food and drink and only the best will do, you know gluttony. And if you're seduced by an inordinate love of giving or receiving flattery and by a deep-seated need to be liked, or by sexual pleasures, this is lust.

--unknown source