An astronomical unit, or AU, and a light-year are two separate measurements that are used for different distances in space. According to the University of Nebraska, an AU is the smallest distance used to measure distances in space, and the light year is the standard measurement for interstellar distances.
An astronomical unit is the average distance between the center of the Earth and the center of the Sun, which is about 150 million kilometers. Interplanetary distances are best measured by the astronomical unit.
A light year, however, is the distance that light travels in one year in a vacuum. Because light travels at approximately 300,000 meters per second, the light-year is approximately 9.4 trillion kilometers, or 63,240 AU. Measuring interstellar distances with AU as the unit would be like measuring the Earth's circumference in feet, which is impractical.
For even larger distances, astronomers use the parsec, a unit equal to about 3.26 light years. When viewing the Earth and the Sun from a distance of one parsec, the angle between them is equal to one arcsecond, or 1/3600 of a degree. Prefixes to a parsec, such as kilo and mega, denote distances in the larger universe. For example, a megaparsec is a million parsecs, or 3.26 million light-years.