Definition of quads
A quads is in French, a square. You get a four of a kind when you have four cards of the same denomination. When a player has a pair of 5's on 5-5-2, for example, he has a quads. A quads is a very rare hand. Even if you have a pair in your hand, you will hit a quads on the flop only 0.24% of the time. And you'll have a pair in your hand 1 time in every 16 hands. That's how rare it is! The good news is that it's an almost unbeatable hand. Even if you play a lot of poker, in your entire life you're likely to lose about 2-3 times with a quad. And we're talking about a player who puts up a good volume!
Quads and badbeat jackpot
Some poker rooms offer progressive jackpots that they call "Badbeat Jackpot". If you lose at one of these tables with at least a pocket 8 (it could be a pocket 9 or 10 or Jacks, depending on the cardroom) and you have a pair in your hand, you will split the progressive jackpot of the moment between all players at the table. The loser of the hand will get 50% of the jackpot, the winner of the hand 25% and all other players at the table will split the remaining 25%.