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NetQuake
From Quake Wiki
When writing about Quake, people sometimes use the terms NetQuake and QuakeWorld.
NetQuake
NetQuake is an informal name for the original Quake engine, or for Quake games played with the same networking code as in the original engine. The original engine had severe shortcomings in its playability over relatively high-latency networks like the Internet.
QuakeWorld
QuakeWorld is an alternative, official Quake engine which id Software released in order to make multiplayer games of Quake more playable over the Internet. Its networking code was rewritten, the in-game physics is slightly different, and there are various improvements in the client. The official versions of QuakeWorld don't support single-player games, but some of the unofficial engines do.
QuakeWorld is also an informal name for multiplayer Quake games played with the QuakeWorld networking code.
Compatibility
An engine will generally either have NetQuake code or QuakeWorld code, not both.
Quake (NetQuake) clients aren't compatible with QuakeWorld servers, and QuakeWorld clients are compatible with ordinary Quake (NetQuake) servers.
Virtually all multiplayer Quake games on the Internet are QuakeWorld games. So if you want to play online, you need to get a QuakeWorld-compatible engine.