A tart is a female who is attractive and has the air of being promiscuous, even if she isn't.
A tart is a girl likey to get disapproving looks from old people.
In short, a tart is more of a tease and a flirt
than, say, a whore
, or slut
. All of those words imply putting out
, wheras a tart may or may not have a lot of random sex.