OK Mark,it goes like this:
In some casinos there is a bad beat jackpot and a small "rake" out of each hand that is played every day goes into that jackpot.
Each casino which features a bad beat jackpot has different rules on how to win this.
Let's say for argument's sake that a casino says that the bad beat jackpot can only be won by a player who holds at least a full house consisting of Aces over Jacks. This means that if a player holds that hand or higher and is beaten by another player with a higher hand then the player who loses(thus the bad beat),will win half of the money that is in the jackpot. Say that the jackpot is $100,000...a nice round number....then the player who lost that hand gets exactly half of that $100,000($50,000). The player who beat him that hand gets half of what is left($25,000),and....depending on how the whole thing has been pre determined....the rest of the players at the table at that time WHO WERE ACTUALLY SEATED AND PLAYING AT THAT TIME would split the remaining $25,000. Some casinos have it set up so that ALL players playing in the room at that time get a share of that $25,000. It just depends.

NOW...usually(always I think),a person who loses/wins the bad beat jackpot MUST have both hole cards play in the hand to qualify. For example:I have A-2 and the flop is AA7 and then the turn is a J and the river is a J. I would not win the bad beat(if beaten) as I am not using both hole cards as the 2 is not in use in the hand.
When the bad beat jackpot is hit(it takes weeks or months sometimes),it starts all over again at a lower prize...say $20,000 and then builds up again.
Some casinos say that you have to lose with quads over quads,some BB jackpots have lower qualifications
Had a friend win one in Vancouver two years ago. She had AA and the flop was AJJ,she bet and the opponent went all in. She called of course and he had quad Jacks.She was annoyed that she lost, but the whole table was clapping and very happy. She had no idea that she had won. She wound up with $13,000. The guy that beat her got $6,500 and the rest of the table shared in the rest. She wasn't annoyed anymore after that.

AS A FOOTNOTE: One of the players at the table had gone off to the washroom before the start of the hand,so was not there when the hand played. HE GOT NOTHING!! That pee break cost him around $800.
Hope that explains it a bit.