Ants (Formicidae) belong to the same order of insects (Hymenoptera) as wasps and bees. They're social insects, with sterile flightless female workers , and flying males and females. The males don't live long; their only purpose is to mate with females that may become queen ants.
Each colony may have one or more queens, depending on the species.
Weird as it may sound, we didn't have any ants in our garden in 2004—at least none that I could find, and I've looked pretty thoroughly. These two are from right outside our front door:
This year, I'm happy to say, we do have ants in the garden, so expect to see some more pictures of them: it's slightly less embarrassing to lie flat on your stomach in the privacy of your own home, as opposed to in the street :)
The ants we have, very imaginatively called “small black ants” (Lasius niger), take to farming aphids for the honeydew they excrete.
I almost feel sorry for the ants on the right: four ants to one aphid, after the Great Aphid Hecatomb of May 2005 (due to a whole rafter of hover fly larvae).