For every generalization I make about biology, there may well be thousands of exceptions.
There are a lot of living things in the world, and precious few generalizations hold true for all of them. I will try to cover the principles; try to keep the existence of exceptions in mind as you read. Another thing to remember is that an important part of understanding biology is learning its language.
Biologists, like many scientists, use technical terms in order to be precise about reference. Getting a grasp on this terminology makes a great deal of the biological literature accessible to the non-specialist. The notes contain information about terminology and other basic matters. With that, let's begin at the beginning.