Prefabs are just models created with the Radiant editor just the same as basic architechture.
The brushes and patches can be grouped as entities to be used later on in an existing map. In Radiant you can then load these models via the edit menu and place them wherever you please. You can even resize or stretch them to create interesting effects within your environment. Feel free to download any of these to your prefabs folder and try them out. All I ask is that if you do use them for a published map, please give me the credit in your text file.