I'll try to do a rough explanation based on two of my favorite works (one of which primarily focused on defining ancient paganism, the other of which focused on characterizing modern Paganism.)
Paganism is actually an umbrella term used to describe a number of unique, yet related religions. The most general description that could be given is that they are at least somewhat loosely based on the practices and beliefs of the pre-Christian religions of Europe (and at least according to some authors like Jones and Pennick, India).
Beyond that, there probably aren't any universal beliefs that every Pagan group holds to. However, there are a rough list of beliefs that are common to the point that any Pagan group will hold to a large number of these "common threads" and each individual belief is held by a large number of Pagan groups.
1. The belief in multiple gods on some level (even if it's a matter of those many gods being "aspects" of a greater Divinity, such as the case in eclecticism).
2. The belief that Divinity is both masculine and feminine.
3. The belief that Divinity is immanent. (Often, Pagans believe that Divinity is both immanent and transcendant.)
4. The belief that the natural world is beautiful, precious, and valuable.
Hrm...For some reason, those are the only four items I can think of at this time. The original list I'm trying to pull from memory (compiled by Gus diZerega in "Christians and Pagans") listed three other "common threads." Unfortunately, they escape me at the moment.
I hope that helps.