Answer:
The basic principle of gambling is that an event has two or more outcomes, and each is given an estimate of how likely it is to be the real outcome of any situation. People decide which they think will occur, and money (or other goods, if you are either not serious or very serious indeed) is placed - this is a bet.
The idea is that should a person pick the correct outcome, they will receive an amount back proportional to what the put in, this proportion usuall based on the odds of that outcome. Hence while an outcome may have a great potential profit, it is not very likely to happen, and while it is more likely to happen, it has a very low profit comparatively speaking.
Most gambling games or events have a system to determine how the odds are created, or these odds might be inherent in the event itself. The event might be one-off, or repetitive; it might be a singleaction, or many actions which correspond to many different combined outcomes.
Gambling can, has and will take place regarding many events: sports games, racing, card and board games, basic skills competitions.. you name it, people have placed bets on it at some point.