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A 'pure server' is a server that forces all clients on the server to use content that matches what is on the server. This way, clients who are playing on the server cannot cheat by modifying the game content.
Without pure servers, clients could modify their materials, sounds, and models to give them advantages in the game. They might allow themselves to see through walls, make enemy models larger or brighter, or make certain sounds louder, thus giving them an advantage while playing.

http://developer.valvesoftware.com/wiki/Pure_Servers