Swimbaits are a loosely defined class of fishing lures that imitate fish and tend to be distinct in design from a typical crankbait.
Swimbaits are usually different from crankbaits by the way they generate lure action. Some are rubber "paddle tail" lures that appear to swim when the tail flutters during retrieve. Some are jointed baits that wave like a flag in the water when retrieved, without any obvious mechanism to generate motion. Some are large jointed crankbaits or crankbait/plastic lure hybrids.
Swimbaits originated as lures designed to imitate the planted rainbow trout in Southern California reservoirs that Largemouth Bass and Striped Bass fed on. They were larger and more lifelike imitations than most available mass-produced lures.
The term swimbait is often used to indicate plastic "paddle tail" lures, regardless of size or appearance.