Gerbils - or at least, the kind that you find in the pet store - are a species of rodentia out of Mongolia.  They're specifically adapted to the desert environment, and dig out huge burrows underground.  As pets and in general, they're cute, fuzzy, curious, friendly, and go around chewing the stuffing out of everything.

They're a social species, so for the "adopting out in pairs" bit that is popular among breeders. In the wild, they usually they live in colonies of Mum, Dad, and all the children who have yet to leave home, 'though in captivity its best to keep them in same-sex pairs (intentional breeding aside).

Of all rodents I've personally had experience with, gerbils make the best pets.  A well-adjusted gerbil is interested in everything that you do, and enjoys trying to make sense of it.  They're very intelligent and can even understand simple gestures and commands if you stick to it long enough.

In short, I find them darling.

I'm no expert, though, so if you want to find out more, head over to the links section or check out the American Gerbil Society.

Once upon a time, I owned a storebought Agouti gerbil with a spot on her head.  Her name was Snowflake, after the spot and in honor of a friend's gerbil I'd known named Snowball.  Anyhow, Snowflake lived for five-six years on top of a shelf, and got very fat on love and mango seeds.  She was very good friends with the cat, although the cat might not agree.

Such a good impact had Snowflake, that around six years later I made the decision to return to a love of rodents in the form of husbandry.  And lo, Xanadu was born.