Wikipedia: Astronomical unit

An astronomical unit (abbreviated AU) is a unit of distance, originally defined as the semimajor axis of the earth's orbit, which is the mean distance of the earth from the sun. For greater precision, the IAU in 1976 defined it as as the distance from the sun at which a particle of negligible mass in an unperturbed orbit would have an orbital period of 365.2568983 days (a Gaussian year). The precise value is 149,597,870.66 km. For the layman, the distance is approximately that of the Earth to the sun.

At the time the AU was introduced, its actual value was very poorly known, but planetary distances in terms of AU could be determined from heliocentric geometry and the laws of planetary motion. Eventually the actual value of the AU was determined (approximately) from parallax observations, and more recently (and precisely) by radar.

Here are a few examples: Jupiter is 5.2 AU from the sun, and Pluto is 39.5 AU from the sun. The moon is 0.0026 AU from the earth.

Some approximate conversion factors:

1 AU = 149,600,000 km = 92,960,000 miles = 490,800,000,000 feet
1 ly (light-year) = 63,240 AU