Led by future-world Hitler Youth club president Cato (Alexander Ludwig), the Careers are the biggest threat to our heroes in the arena. They come from District 1 and 2, where (as Haymitch's helpful plot exposition tells us) they play to win.
HAYMITCH: He's a Career. You know what that is?
KATNISS: From District 1.
HAYMITCH: And 2. They train in a special academy until they're eighteen, then they volunteer. By that point, they're pretty lethal.
That's really all we need to know about them. The rest we can see by their behavior: Cato's intimidation of some poor schlub during training, for instance, or the very existence of Clove (Isabelle Fuhrman), who seems to have honed her Skunk Eye to lethal proportions.
Their ranks also include Glimmer (Leven Rambin), mean girl of mean girls, and her partner Marvel (Jack Quaid) who's got this unsettling spear thing going. He's the guy who kills Rue, which turns out to be a very bad move for him. They're joined by a young kid from District 3 (never named, but played by Ian Nelson), who basically stays with them to keep from dying. Peeta forms a pseudo-alliance with them for the same reasons, though he proves to be more loyal to his would-be sweetie Katniss than the thugs he's surrounded himself with.
Anyone who's ever been on an elementary school playground can spot their swagger a mile away. They're bullies, and the arena is just their personal mosh pit.
But in the end, that's all they are. Like bullies, they can be dangerous if you don't stand up to them, but like bullies, their power is still very petty and small. They're still sacrifices in the arena. While they talk tough as a group, sooner or later they're going to end up killing each other. The real villain is Snow, and like all good minions, the Careers are only there to live and die at his command.
That said, we still don't feel for them. They called the tune, they get to pay the piper.