Definition of bounty
A bounty is a bounty placed on someone's head. At the time of the cowboys, a bounty could be put on the head of the man to be shot. There was a poster with the head of the man in question with the words: "Wanted, bounty of 10,000$". At poker, a bounty is an amount on a player's head. Generally speaking, the bountys is reserved for tournaments. They vary depending on the limit you are playing at.
Example of a bounty tournament
You spot a bounty tournament on your online room of poker. The fees associated with the tournament are as follows: 10+5+2. The structures may be different, but this example will approximate what you will find. The first number (10) is the amount of the tournament buy-in. This money will go directly into the main prize pool just like any other regular tournament. The 5 is the amount that will be on your head. Each player will pay 5 euros which he will have on his head. Every time you eliminate a player, you will directly pocket his 5 euro, even if you don't win the tournament. Each player you eliminate will allow you to pocket 5 euros. Finally, the 2 is the commission that the room of poker will take from the tournament. If you have eliminated say 5 players during this tournament, it will be said that you have collected 5 bountys. Only the player who wins the tournament will be able to pocket his own bounty.