The difference is using your with instead of being brutally obvious, playing with your hair and pushing your boobs out.
Classy flirting would be more subtle. Its not as obvious but it makes the opposite gender think. A smile. complimenting on how one carry's themselves. e.g "You are very easy to talk to" "You are a really fun person to be around" You can also compliment their style, looks and anything about their character.
Eye contact and a smile would indicate interest, but have open body language. Meaning be relaxed and comfortable around the object of your affection.
"slutty flirting" would probably be consistent of a lot of groping and grinding. Throwing your assets out in plan view with intent to show. Grabbing at a man and talking about how your legs would look great over his chest.
Either of these two methods will work on the common folk. Id go with classy flirting if you are in a workplace or anywhere that is not a bar/club.