Prefab
A Premade Fabrication or Prefab is a separately saved object, usually consisting of multiple parts, either brushes, props, or entity elements. The prefab system allows you to easily store pieces from your levels that can be used repeatedly. Examples of useful prefabs include hallways, pillars, special entity setups, etc.
Prefabs are simply small map (.VMF) files that contain only your prefab. These files are stored in a particular prefab folder, generally \sourcesdk\bin\prefabs, so that Hammer can find them and allow you to easily insert them into your levels. Prefabs can be organized into their own heirarchy in the /prefabs directory - Using a folder of prefabs for each map, mod, etcetera, as determined by the user.
To create a prefab, select the objects you wish to create it from and select the Create Prefab command from the Tools Menu. Hammer opens a Save As... dialog open in the Hammer prefabs directory. Type in a filename and click the Save button. The prefab will now appear in the New Objects toolbar. When creating a prefab that must interact with named objects inside itself, such as a button that triggers a door to open, place "_&i" at the end of each objects name so that the prefab can be used several times within a map without the individual prefabs activating each other.
Note:When creating prefabs, it is generally advisable to centre the objects at the origin (0,0,0) before finalising the prefab.For information on how to place prefabs in a map, see Inserting Prefabs.
External links
Various sites that host custom made prefabs and models: