Hello. There are a multitude of foundation types, varying based on the loads they carry, and the construction type they support. The first important differentiation is between residential and commercial architecture. For residential structures, there are two basic types of foundation: Linear spread footings and "slab-on-grade concrete. Linear footings are made from reinforced concrete ("Rebar" is short for "Reinforcing bar") Rebar adds tensile strength to concrete which has virtually none (imagine how easily concrete can crack). The reinforcing is critical to any structural element to add tensile strength. So a linear reinforced footing is shaped like an upside-down "T", with the "stem" exposed above the ground, and the structure sitting on it. Slab-on-grade is, again, reinforced concrete, but is a much simpler construction type, basically a 6" slab of concrete with rebar in a grid pattern, and poured out to a fixed size and shape. (Think of a single-family residential garage). There are very simple "foundations" used to support things like mobile homes, etc, using concrete blocks, but these are not actual foundations. For commercial architecture, linear spread footings can be used for smaller one or even two story structures, but usually the foundations are much more robust. There are piers, which are basically huge long/tall tubes of reinforced concrete or steel "I" beams that are forced into the ground to a depth that can vary anywhere from 2 to 100 feet or more, depending on the need. For super high-rise buildings, even piers are not enough to support the millions of tons bearing down on them. This requires highly specialized types of foundations like stem foundations, and others.