What is a Astronomical Unit?
A Astronomical Unit is a calculated distance that splits between our planets and stars. The average AU is 149,597,871 kilometers. What we use with an AU is to measure the distances in space. Let's take the Earth for an example. It is one AU (Astronomical Unit) away from the Sun. Since we can't use miles or kilometers in space, we use AU's because it's only meant for space and nothing else.
What is a Light Year?
A light year is another way we measure things in space. But instead of planets, we use it to measure light in space. Let's just say a supernova. Since stars are far away from us, the light of the explosion doesn't hit us until a million years after the explosion. The length of a light year is 9 trillion kilometers. A fast ride, huh? So the Speed of Light is the fastest thing on Earth. Sometimes we use it to measure distance.
We also call all the Light Years "The Speed of Light" The speed of light is 299,792 kilometers per second. A whole bunch!