What is the difference between confined and unconfined aquifers?

Unconfined (water table) aquifer has a free water surface open to atmosphere, bounded below by impermeable layer. Water level rises and falls with recharge. Confined aquifer is bounded above and below by impermeable layers, water under pressure. When tapped, water rises above the aquifer top; if pressure sufficient, artesian flow occurs. Unconfined aquifers are more susceptible to contamination but easier to recharge; confined aquifers provide more consistent water quality but recover slowly.