The foot is a unit of length in the imperial unit system and uses the symbol ft.
One foot is exactly equal to 12 inches. It is also exactly equal to 0.3048m. One yard is comprised of three feet.
The foot is just behind the metre in terms of widespread use due to its previous popularity. The US is the only developed country that still uses the foot in preference to the metre.
The UK still uses feet to express human height more than metres.
The astronomical unit is a unit of length and is used in the Astronomical system of units and denoted by the symbol au.
It represents the average distance between the Earth and the Sun which is approximately 150 million kilometres (or 93 million miles).
It's used mostly to express distances within our Solar System.