Scuba diving is an activity where people (called "scuba divers", or simply "divers") can swim underwater for a long time, using a tank filled with compressed air. The tank is a large metal cylinder made of steel or aluminum.

The word scuba is an acronym from  Underwater Breathing  Apparatus.

History
In the 1600s, a diving bell would be lowered that had trapped air in it. A diving bell is like a large heavy upside-down bucket that holds air inside when lowered into the water. A diver would breathe that air and swim in an and out of the bell to work until the air became bad. Later fresh air was pumped to the diving bell through a hose, so the diver could stay longer.

The first diving suits used a heavy copper diving helmet with windows and a hose from an air pump. The helmet was clamped to a waterptoof diving suit. Divers would wear heavy weghts and walk on the seafloor as it was not safe to swim.  The air hose would send air to the helmet from a pump on land or on a boat above the diver. The air would pass trough the helmet and out into the water through a valve, and the diver would breathe from the air in the helmet. This wasted a lot of air, but there was no way to only supply air when the diver needed it. This is called a free-flow system.

Jacques Cousteau was a Frenchman who developed several important parts of the scuba system and made it useful.  One part was a better regulator that only sent air when the diver breathed in.  This let the divers go farther on one tank.  It was made with a mouthpiece on a rubber hose, so a heavy helmet was not needed. It was light enough to use with fins and easily swim.  He also took many underwater movies and showed people what was under the water and why it needed to be protected.

There have been improvements to scuba equipment since Cousteau to make it safer and easier to use.

Equipment
 Scuba tank, (one or more) containing air.  The air is ordinary compressed air for most dives, but for longer dives a mixture of air with more oxygen, called nitrox can be used to avoid decompression sickness, also called the bends (a painful or deadly problem from going to the surface too fast). The tank is attached to the diver by a harness, which is often part of the BCD. For deeper dives a breathing gas containing helium can be used to prevent nitrogen narcosis, which can make the diver behave as if drunk, and is dangerous.
 Regulator for breathing the air from the tank.  This reduces the pressure of the air coming from the tank. It adjusts the air to the pressure of the water around the diver so they can breathe easily at any safe depth. There is also an air pressure gauge to show how much air is left in the tank.
 BCD (buoyancy control device) to control whether the diver floats or sinks.  The diver can add air from an inflator valve or remove air by opening a dump valve. Divers may also wear weights to stop themselves from floating upwards when their diving suit is too buoyant. The BCD lets the diver adjust their buoyancy so they can be neutral (float in place) at a depth they want.  If the diver swims upward the air in the BCD will expand because the water pressure is less, and the diver must let some air out or float to the surface. If the diver swims down the air will compress and the diver must put in more air or sink to the bottom, so the diver must carefully control how much air is in the BCD whenever their depth changes.
 Depth gauge, to know how deep they are and there may be a watch to tell the time or a dive computer to show how slowly they must come up to be safe from decompression sickness.  A safe dive depends on how long and how deep the diver goes, and how long it has been since the last dive.
 Most divers wear a diving suit to keep warm. This is usually a wetsuit made from foam neprene rubber, but for colder water a dry suit can be used.
 Divers wear a mask to see through, because the human eye cannot focus properly in water.
 Swimfins are worn on the feet to swim better. 
 A snorkel can be used for breathing at the surface when swimming face down

Certification
Divers face dangers that can hurt or kill them if they don't know what to do.  They can use up all their air, or get decompression sickness or nitrogen narcosis.  A person must be trained to use the equipment and dive safely. When they show that they can dive safely they get a certification card.  The biggest organization for certifying divers is PADI - Professional Association of Diving Instructors - but there are many others, depending on the country.  Some tourist places have a short course without certification and then the instructor will lead the diver in a shallow dive, all in one day.

Because of special dangers, there are advanced classes for things like diving in or around underwater shipwrecks, cave diving, and deep diving (more than 60 feet or 18 meters).

Related pages
 freediving - Diving without scuba equipment (holding your breath).  Snorkeling equipment is often used in freediving.  No certification is needed but it can still be dangerous.

References

Diving