Under the new definition, the astronomical unit (or AU) — the measurement used for the Earth-sun distance — is no longer always in flux, depending on the length of a day and other changing factors. It is now a fixed number: 149,597,870,700 meters, which is the equivalent of almost 92.956 million miles.
http://www.space.com/17733-earth-sun-distance-astronomical-unit.html
Basically: light years or parsecs for interstellar distances, AUs for interplanetary distances, and the more familiar measurements are for local scale.