With the rising of computational power across the network, it has become progressively more difficult for individual miners to mine coins, and mining pools have emerged as a way to accumulate the computational power of a group of miners together.
Miners simply connect their machines to a pool's online servers to pool their computational power with other miners, thereby increasing their probability of finding new blocks. Once the mining is successful, each miner in the pool gets his share of the reward, depending on the miner's contribution and the pool's rules.