There are two basic types of dwarf hamsters currently available as pets: the Russians and the Chinese. The Russians can be further divided into the Campbells dwarf hamster, also known as the Cjungarian or simply as the Russian Dwarf, the Siberian or Winter-White, and the Roborovsky. One cannot keep Chinese and Russian dwarfs together, but any of the Russian group may be kept with others of the Russian group.
The Chinese Dwarf Hamster is considered to be one of the "ratlike" hamsters. They have a slightly longer tail, more pointed and longer nose, and tend to be thinner in build than other hamsters. All in all, they do look more ratlike. They are brown, with a dorsal stripe.
The Chinese hamsters all have a dorsal stripe. The Campbells is brownish-grey in color, with a white belly. The Siberian or Winter-White is grey in color during the summer and turns almost entirely white in the winter--the change being triggered by the length of days, not by temperature. The Roborovsky is similar to the Campbells, but slightly darker, and has white patches above the eyes, resembling eyebrows.
Dwarf hamsters are much more sociable than the more common Syrian hamster. They will live in pairs or small colonies and will friendly towards humans. The female is dominant and females will sometimes fight. Otherwise peaceable females can become highly territorial while pregnant, which may include attacking her mate and/or you.
Russian dwarfs are relatively easy to breed, Chinese somewhat less so. Generally their litters are small, 2-6 babies. The female goes into heat and can become pregnant immediately after giving birth. The male will attempt to be a good parent, although the female may not allow him to.