AT-AT stands for All-Terrain Armored Transport.
Lego AT-AT walker:
Episode V AT-AT walker:
The thing about AT-ATs is that they are just so darn adorable. They resemble little dogs, and, in fact, there are many artworks and t-shirt designs to that effect:
AT-AT walker as a dog drinking from toilet:
AT-AT walker as dog, being walked by Darth Vader:
Then there's the AT-AT Walker walker, which is how to use a walker in style:
When I saw the ads for this year's Lego Star Wars advent calendar, the first thing I did was to create my own AT-AT walker from my spare Lego bricks. It's a great build, and instantly recognizable.
Now the only question is who would win in a battle, an AT-AT Walker, or a mumakil?
For more information about the AT-AT walker, see Wookieepedia.
For parts information for this build, see Bricklink.
For more information on the mumakil, see the LOTR wiki.