In an aquarium, we rely on water to deliver oxygen & trace elements to organisms, and waste to skimmers & filters. Good circulation is absolutely essential, and is a major concern in large or heavily stocked tanks, which can develop stagnant "dead spots". Water movement also discourages algae, and promotes fish health by forcing them to swim harder (stick to low flow rates for fry, live plants and stagnant water fish that aren't used to exercise).
Aquarium water pumps circulate large volumes of water; powerheads are small pumps that produce currents. Different aquariums have different circulation needs, so browse the sections below for more information, or check out our full-length article on Proper Aquarium Circulation.