Most Helpful Guy
Okay, this is a little hard... here, I'll try to explain. Certain types of clothing seem slutty, while other types show off tons of skin but don't seem. Ever seen the Jessica Simpson music video for "These Boots are Made for Walking"? She's got like short jean shorts on, etc. That outfit is sexy, but doesn't seem as slutty. Or like Mila Kunis in "Max Payne." She's got some neat black outfits that are sexy but don't seem slutty.
Now, or opposite examples.
If you've seen "Hitch"? There's a scene in a bar where Hitch gets a girl to follow him by handing her cash and ordering a drink. Her outfit is classy, but slutty. Too much cleavage.
To try to summarize it, it's slutty when it's too obvious you're deliberately trying to show off your breasts or your butt or whatever.