All miners connected to the pool are mining for the pool. So if anyone finds a block the block will belong to the pool (gets paid out to the pools address).
Now the pool has the responsibility to split the reward (25btc currently) between miners depending how much work (hashpower) they put in to find this block. Usually this is done by pools setting a lower difficulty for the miner when sending them jobs. Miners will send back results (called shares) earlier when they find a block matching the lower difficulty set by the pool. The pool will count this shares as proof that the miner actually did the work.
Shares are like real blocks that just don't match the full target (difficulty). One of those shares will actually match the current real difficulty and the pool will submit it as a valid new block.
Different pools use different ways to count the shares (determine your payout) and you will see terms like PPS, PPLNS, ....
You can find more information on wikis:
https://en.bitcoin.it/wiki/Mining_pool_reward_FAQ
https://en.bitcoin.it/wiki/Pooled_mining
https://en.bitcoin.it/wiki/Why_pooled_mining
The hashrate is on a steep curve going up right now and it looks like each week a new mining data center is announced. I personally would wait a bit now before investing into mining hardware.