A pool combines the hashrate of many miners and distributes rewards in proportion to contribution. When choosing a pool, look at the fee (typically 1-4% for large pools), minimum payout, and in which currency it is paid.
Pool reliability is judged by uptime, reviews and track record. Server geography affects ping: the closer the server, the more stable the connection and the fewer stale shares. Some pools offer bonuses for consistent mining or different payout schemes (PPS, PPLNS, FPPS) - this determines how your share is converted into payouts.
It pays to calculate net profitability after the pool fee and compare options using the calculator on the main page.