Here is a GREAT article addressing many of the common myths about ferrets like:
Aren't ferrets related to rats?
Descended from the Siberian or European polecat, ferrets are mammals but definitely not rodents. They are members of the weasel family and are related to minks, ermines, otters, skunks, badgers, and wolverines. (It's true that these wild cousins are likely to eat a rat.) The scientific name for the domestic ferret is Mustela putorius furo. Read more...