Well that last part of the url Antiques__W0QQ_sacatZ20081 you've probaly worked out is a parameter string, I'd say the parameters are separated by underscored - the second one being blank. The order is predefined so it's easy to attach them to variables.
Mod Rewrite (google it) is used to convert it to basically the equivalent of:
items.php?cat=Antiques&something=&subcat=W0QQ&id=sacat720081
After that it's just all php. The mod rewrite thing might be the most confusing thing if you've never seen it before, but there's plenty of examples of it on this forum if you search.