re: Poker & Prop Bet, Side Bet and Crossbooking
"Prop bet" is short for "proposition bet" and it's, quite simply, a bet made on a proposition. As bigjoker notes it's often on something trivial or stupid. Things like Erick Lindgren's famous "shoot four rounds of golf under 100 in a day" bet fall into this category. The bets people make during a poker game
on what the predominant colour or suit will be on the flop probably fall into this category too.
A side bet is one made on the side of a game - say a last longer bet in a tournament. It could in theory be a prop bet too, I don't think the distinction is all that important.
Cross booking is very different though. It's a specific type of bet where participants agree to pay each other the amount that the other wins in a tournament. If we agree to cross book in a tournament, you fail to cash and I cash for $300, then you owe me $300. If you cash for $500 and I cash for $300 then I owe you a net $200.
What bigjoker is talking about is actually called swapping pieces - each player owns a certain percentage or "piece" of the other's action.