Any good for nothing asshole, may be a dude or a chick, who woos your partner out of a relationship with you. They've also been known to drop out of the dates quickly, and steal someone else's date. More than likely these people are immature idiots who don't know how to find themselves a meaningful date, so they have to steal someone else when they see a flaw in a relationship.
The Family Guy episode about Peter Griffin's jealousy of Louis seeing other men, there's that quick joke in the theater when Louis says "Ain't he handsome!?" Peter runs up and punches the screen until it breaks, screaming "You home-wrecking bastard!"
Country singer Gretchen Wilson has had a lot of experiences in high school with losing her dates to homewreckers, so she included the subject in a song on her album "Here For The Party" and called it "Homewrecker."