According to the law, an annulment is a court procedure that like a divorce dissolves a marriage. But unlike a divorce, an annulment treats a marriage as if it never happened.
Nolo, an excellent guide to the law that even a layman can understand, differentiates civil vs. religious annulments. Furthermore, the site explains divorce (including no fault divorce) in detail and separation.
Related articles discuss how to find a divorce lawyer and how property division and alimony is determined in your state.
For more on annulment and divorce, check out 50somethinginfo.com.