Given the extremely high difficulty of Bitcoin mining now, it may take months before an individual mining rig manage to solve and receive a block of 50 Bitcoins. Therefore sites started to appear which pool together the hashing power of the individual rigs so as to combine them for Bitcoin mining and based on the hashing power, each time a Bitcoin block is solved, the 50 Bitcoins are shared among the miners. Therefore payouts are much more frequent especially in the bigger Bitcoin Pools.
Bitcoin pools do require resources such as servers and bandwidth to operate not to mention the manpower involved in maintaining the Bitcoin Pools, therefore most of them charge a small percentage of the Bitcoin produced as their fees. But I feel that Bitcoin Pools are a Win-Win situation unless you are a large Bitcoin Rig operator able to do Solo Mining with enough hashing power to produce the Bitcoins on a regular interval.