In astronomy we measure the brightness of objects in different ways. This is because the way we see objects is affected by our distance from them, as well as how bright they actually are, known as their intrinsic brightness. We therefore use the term magnitude to describe the brightness and the units are very unusual, with the brighter objects having a lower (sometimes negative) number. When we discuss magnitudes we describe an apparent magnitude, which is how bright the object appears from the Earth, and an absolute magnitude, which is how bright the object would be if it were placed at a set distance from us. This distance is defined as 10,000,000 parsecs (or 10 mega parsecs), where a parsec is the same distance as 3.26 light years, or 31,000 billion kilometres (19,000 billion miles)!
Sometimes we also use a measurement for brightness known as an objects luminosity which is more closely related to the brightness we can measure, which is related to it's energy.