A fuckboy is a guy who plays with many girl's hearts at the same time. He may be dating one girl and secretly going out with another.
They're hated, because it's just like being a whore, except you're a male. Like cmon, who likes being lied to and cheated on/played with?
And why do girls get with them anyway? That's a good question. Most girls are wired to have an Alpha. Like an alpha wolf. The fuckboy has many female followers because he's charming/handsome/rich/blah blah blah They just can't resist the tug/desire to be with someone who's important/well known.
I think it's one of those terms that is applied to whatever group of guys people feel like. I've seen it used before in a bitter article complaining about how girls don't go for "nice guys" to describe any guy a girl went for. I've also heard it used to describe the douchebag-frat boy stereotype. If I had to put a rough definition on it, I'd say it's used mostly to describe the "bro" type of guys - drinking, partying, working out, etc. That's the best I could say.
I've always considered it a Justin Bieber type. Basically trying to be someone they are not by dressing like a douche. Some guys consider fuckboys anyone who dresses remotely well, takes care of themseleves, and doesn't only own one pair of cowboy boots. Girls get with the second type, not the first so much.