Hydration is a measure, expressed as a baker's percentage of how wet a dough is. The hydration includes all the flours in the dough, including any in preferments, and all the liquids and fats in the dough, including any in the preferments.
While hydration gives you an idea of what a dough will look like, it's not terribly useful all by itself. Whole grain and high protein flours absorb more moisture than lower protein and refined flours. So, you need to look at the kind of flour as well as the hydration.
An 80% hydration white bread is pretty wet, where an 80% hydration whole wheat bread may be a bit too dry, depending, of course, on what sort of bread you are trying to make.
Traditionally, bakers haven't included oils and fats in the hydration calculations. However, at Camp Bread 2007, Craig Ponsford stunned a few of us when he told us he felt oils, eggs and water were largely interchangeable for hydration purposes. Despite initial skepticism, I've found he seems to be right. Then again, Craig is a world class baker and if you think he's wrong, that invariably means it's time to re-examine your position.