Air cylinders used for scuba diving come in various sizes and materials and are typically designated by material (aluminium, steel, high-pressure steel, etc) and by how much air (in cubic feet) they contain at 1 atmosphere (80, 100, 120, etc). The most common is the "Aluminum 80", which will give an average experienced diver from 40 to 60 minutes of dive time under common dive conditions.
Air cylinder pressure will vary according to the type of material used, ranging from 2000 psi up to nearly 4000 psi.
Aluminium cylinders are less expensive than steel and have been known to last for 20 years with standard regular maintenance. The drawback is that aluminium cylinders are neutrally buoyant when full, and positively buoyant when nearing empty. This results in having to monitor buoyancy during the dive more closely so as not to experience the "express elevator to the surface". Aluminium tanks also need the diver to carry more weight. Divers often prefer to use steel tanks as they are negatively buoyant when full and neutral when empty. Many steel tanks also accept higher pressure fills, giving more capacity for a longer dive for the same size of cylinder.