Ah, I think you mean prefabs. They're just brushwork that's been premade and saved as a group. They're kind of frowned upon, as you can tell if the mapper has put prefabs everywhere in a map, because you see the same thing in lots of other maps.
(Of course things like file cabinets and the like are fairly standard stuff, so you may as well use them. Just don't use them too much.)
Also, invalid prefabs will cause Hammer to crash, and there are plenty of invalid ones around. I'm pretty sure that more recent versions of Hammer don't include any prefabs.
If you really want to use the prefabs, find an older install of Hammer (such as 3.3, not sure if they're included), or a copy of Worldcraft 2.1 (definitely included), and copy the 'prefabs' folder to your Hammer install directory. Alternatively, a Google search might turn up some prefabs.