Answer:
AU is an abbreviation for a number of things. It can refer to Australia, the Angstrom Unit, the Astronomical Unit, and the Arithmetic Unit. In addition, it can refer to Auburn University and Air University.
"Au" (where the "u" is not capitalized) is the chemical symbol for the element gold.
Sometimes, the letters "a" and "u" are not capitalized when denoting Astronomical Units, although prevailing usage seems to indicate that the abbreviation "AU" is more common.