You put your computer to work solving puzzles and earn Bitcoins. Unless you have a really fast computer, or are very lucky, you shouldn't expect to get much.
Most people do it as part of a "pool", which means you cooperate with other people and split the profit. Thus you get a steady trickle of fractional Bitcoins instead of very small chance of 25 at once.
It's not TOO hard to set up.
Bitcoin mining is just like a production machine of different commodities. As there are different machines for different things, bitcoin is also a machine for mining bitcoin. Bitcoin is not a physical currency therefore making of bitcoin is called mining and not production. Mining is also very profitable and covers all the capital of miners in just a few months. Mining also do not take too much time.