Definition of bounty
A bounty is a bounty placed on someone's head. In the cowboy days, a bounty could be put on the head of the man to be shot. A sign would be posted with the head of the man in question with the words: "Wanted, $10,000 bounty". In poker, a bounty is an amount on the head of a player. Generally speaking, bounties are reserved for tournaments. They vary depending on the limit you are playing at.
Example of a bounty tournament
You spot a bounty tournament at your online poker room. The fees associated with the tournament read as follows: 10+5+2. The structures may be different, but this example will be close to what you will find. The first number (10) is the tournament buy-in amount. 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 out €5 on their head. Each time you eliminate a player, you will pocket their €5 directly, even if you don't win the tournament. Each player eliminated will give you 5 euros. Finally, the 2 is the commission that the poker room will take from the tournament. If you have eliminated, say, 5 players in this tournament, then you are said to have collected 5 bounties. Only the player who wins the tournament will be able to pocket his own bounty.