These were the super-giant tanks that the Jawas used in Star Wars. They're basically giant rolling cities that roam the deserts of Tatooine. They run on two rows of giant treads, and the body of the sandcrawler is angular and blockish, but there are different models. Some of them look like they were built to be more aerodynamic, or maybe it's a pointed nose for ramming.
The only major apperance of a Sandcrawler in the movies was in A New Hope, when Owen Lars buys C-3PO and R2-D2. One also appears very briefly in Star Wars: Episode I - The Phantom Menace.
They are armored to protect from Tusken Raiders, treaded to handle any sand dune, big enough for a town's worth of Jawa to live in comfortably, and they contain enough cargo space to support a race of traders and salvagers. They were originally designed to be mining vehicles, but when the Tatooine mines turned out to be worthless they were abandoned to be salvaged by the Jawa.
They are one of the coolest land vehicles in the Star Wars universe, but they are usually ignored because they are so slow. They are also too big and complex to make into a marketable toy, although you can buy models, and Lego used to sell a Sandcrawler set.
DejaMorgana says: There was actually a sandcrawler in the original line of Kenner action figures. It was about the size of the Falcon - way off true scale, of course.