To steal is to raise with a hand less valuable than what might normally be considered a raising hand, normally a below average one, with the hope that the few players remaining will not have a hand worth calling the raise, thereby winning the antes or blinds without further action. This play is used either in late position after several people have folded, or when the game is short-handed. Steals happen most often in tournaments due to the escalating ante/blind structure.
While steals like this don't win much money per hand, they can accumulate to considerable profit if the players to the left of the stealer are tight enough to not contest many steals. Of course, skilled players will recognize repeated steal plays and frequently reraise for defense.
Steals being made in late position or when the game is short-handed are the most common steals, but a raise under other conditions can act as a steal also. An aggressive player, especially one with a large stack of chips, might reraise someone he knows might be trying to steal. The objective here is twofold. The re-raiser hopes to pick up the blinds and antes and the raiser's chips when the raiser folds. And the reraiser hopes to keep this player from constantly raising before she or he can act because that cuts down on the reraiser's opportunities.