There are two common types of stockpots: aluminum and stainless steel with an aluminum disk in the bottom. The Cuisinart that Geo linked to is an example of the latter and that's a great price. It's basically the same as the $150 All-Clad, though the All-Clad comes with a pasta strainer and a steamer basket, and is sometimes advertised for $100.
The advantages of aluminum are that they are cheap and light. You can find these at restaurant supply stores and, of course, on-line. The disadvantages are that aluminum can warp if it gets too hot and it can react with acidic ingredients, such as tomatoes. So if you plan to use the pot to make large batches of tomato sauce, you may want to avoid plain aluminum.
Stainless steel with an aluminum core looks prettier, is heavier, is non-reactive, and is more expensive. So why have the aluminum core? Stainless steel is not a good conductor of heat. So manufactures sandwich a layer (or multiple layers) of aluminum, copper, or other material inside of it to conduct heat. In the case of a stockpot, the base of the pot will have a layer of aluminum, which is a very good conductor of heat. This makes it good for browning foods in oil before you add water to the pot.
Anodized aluminum is a middle ground - it is aluminum that has been treated so that it doesn't react with acid in foods.
lougord99 is basically correct that once the liquid is in the pot, most pots perform pretty much the same.
There are other materials (ie. copper, as budrichard mentioned).
For a home kitchen, I think you'll do well with either a 12 or 16 qt pot.
I can't speak specifically about the All-Clad stock pot (I assume it's the same as the rest of the product line), but the reason their other cookware is prized (and priced) so highly is that besides having a disk of aluminum in the bottom, the aluminum is carried all the way up the sides of the pan, thus heating more evenly and avoiding hot spots. Hence the name All-Clad.