Calvinism is hard to define. You can begin by saying that it is the collected theology of John Calvin, but it isn't really. It's the collected theology of John Calvin plus the thought of many theologians and Christians that follow his ideas.
The core of his theology can be found in Calvin's Institutes of the Christian Religion. The Institutes are a vast sprawling work that draws from much of Christian theology (up to that point). Despite the best efforts of Calvinists, it is hard to explain what Calvin believed in a way that was simple and understandable.
Calvin was a lawyer by training and that influence comes out in his theology. Rather than coming out with a bunch of new ideas, Calvin's achievement is taking many Christian ideas and putting them together in one systematic work. None of the ideas are particularly original. The combination is.
Because of this, the theology is complex and hard to explain simply. People try. T.U.L.I.P. (see the node next to this one) is a good example of this.
In the end though, T.U.L.I.P. merely explains Calvin's ideas about salvation with an emphasis on predestination and thus on how Calvin differs from most Christians--most Christians don't believe in predestination.
What does T.U.L.I.P. miss? A lot. The Institutes are two books. Two THICK books.
Beyond theology, Calvin uses the Institutes to address Christian's relationship to government. Should they follow it unquestioningly or rebel if it oversteps it's bounds? Calvin addresses the Sacrements (in Calvin's view there are two, Baptism and Communion). It addresses Calvin's view of the Trinity, of Christ's human/divine nature, and many other questions.
With all these views and opinions to choose from, people tend to focus on the most important ones (in their view) and call them "Calvinism."
It's because of things like that that you'll find that even many Calvinists do not in fact agree with Calvin.
For example, later Calvinists were strongly influenced by medieval theology, listing various qualities of God that Calvin didn't really consider in so many words--things like omnipresence, omniscience and so on.
These people's contributions are also Calvinism.