Sexy is subtle, smart and almost effortless. Sexy knows what it's doing, but doesn't want you to know that. Sexy doesn't care what you think - sexy is as sexy does.

Slutty, on the other hand, cares very much what you think. Slutty is in your face, to the point of causing offence. Slutty values your opinion of it more than its own.

Sexy is secure and sure of itself, whereas slutty is not. Slutty will put itself through pain for your perceived pleasure, whereas sexy realises how futile that would be. Sexy doesn't realise it's sexy, but slutty tries too hard to be sexy. Slutty's intentions are plain to see. Sexy is more enigmatic.

But this is all based on appearance. Slutty is probably really sweet underneath, and just going about it all wrong. Sexy might be a manipulative bitch. First impressions don't mean squat.