to add to all the things that have been said already, i tend to look at the devil as the rational mind being overcontrolling, our way of rationalizing anything, making excuses for all our own actions. it's when we've stopped listening to our conscience and intuition and are doing things that might not be too good for us.
we could also look at it as "the devil made me do it." passing blame to someone else, or not taking responsibility for our own actions.
or facing our inner demons. realizing that we aren't always going to be the perfect little angel mom might like us to be ;} we all have our darker sides, and this is a reminder to us that they are a part of us, and we should bring them to balance with the lighter side.
the opposing card to this one is the lovers, which is, to me, about following your heart too much without listening to your head. our conscience is a combination of our conscious and subconscious, our head and our heart, and when we are out of balance, we don't listen to it as much. we have two sides (at least) to ourselves, and we need to bring them into balance.
remember also, that sometimes, going from one extreme to the other can be a form of balance ;} excess isn't always a bad thing. just always take things in context and do what you feel you need to do.
