As in what exactly? People use the term for different things, actions and categories of people. It's often confusing to see different people arguing while having different definitions. Some also somehow change their definition every couple sentences.
There are a variety of people who have pedophilia. The causes are unknown, but in some cases you can determine influences (eg. some of those who abuse have been abused or similar) and find correlations.
I think you can categorize groups of them by how it affects them, what it is precisely in them, if they have fantasies or what fantasies they have/could have, what they do about it, and what general background they have.