What Breed Hamster Do I Have?

There are five main breeds of hamsters available in United States pet stores: Syrian, Campbell's Russian Dwarf, Winter White Russian Dwarf, Robovorski, and Chinese Dwarf.
The Syrian hamsters are easy to identify. They are the largest of the hamsters, about six inches long. There are many varieties that are really just different colors and fur lengths, such as the Teddy Bear, Black Bear, and Dalmation hamsters. Despite how they live in pet stores, these guys are SOLITARY and will fight. They live two to three years.
Campbell's Russian Dwarves are the most commonly found dwarf hamster in pet stores. They are small--about three inches long, and somewhat plump. They are social and like to live in pairs or groups, but be very careful when introducing new hamsters. They get along best if they've grown up together, but I know from personal experience that it is sometimes possible to introduce new adults. They live one to two years.
Winter White Russian hamsters are slightly smaller and, most importantly, they turn white during the winter. They have a more prominant face and can interbreed with Campbell's, but they are less common in pet stores.
Robovorski hamsters are very small, only two inches long. They are extremely active and fascinating to watch, but they do not make good pets for someone who wants to be able to cuddle with their hamster. They are friendly and rarely nip, but they certainly will not sit still. They live about three years and are not common in the United States.
Chinese hamsters are small (about three inches) and mouselike. They are timid and quick, making them difficult to catch and play with. The males have very prominant scrotal sacs. There are restrictions on these in some states (California, for one), and can only be obtained from specialist breeders.

Here's a useful website with a picture of each breed: Petwebsite.com - Hamster Species