SCUBA, which stands for Self Contained Underwater Breathing Apparatus, is a device developed by the famous French oceanographer Jacques Cousteau to allow greater freedom during underwater excursions. Previously, dives were done with big helmets that were attached to pumps at the surface, tethering the diver to a surface ship.
SCUBA gear is essentially an air tank with a pressure regulator that you can breathe through. Quite a bit of development went into the design of SCUBA gear, and much of the testing was done by the then-younger Cousteau. On one of these experimental dives, he discovered the toxic effects of pure oxygen at great depths. This lead to the current practice of adding helium into the gas tanks for deep dives.
For simple recreational diving, SCUBA gear usually consists of the tank, a regulator, a mask, weightbelt, and fins. More involved divers shell out money for things like wetsuits, drysuits, BCUs (Buoyancy Compensator Units), and other stuff.