QuakeWorld is an Internet multi-player specific version of Quake. While the original version of Quake can be played over the Internet, many users modem users - the majority of players, had less than satisfactory play. Symptoms like excessive lag - actions actually happening much later than you did them; packet loss - the game would freeze and resume several seconds later; and various other difficulties plagued users.
After realizing how many people played Quake on the internet, and how many wanted to, but couldn't due to the play being unsatisfactory, John Carmack of id Software decided to create a version of Quake that was optimized for the average modem Internet player. This Internet specific version does only 1 thing, play deathmatch games over a TCP/IP network such as the Internet. It has no support for solo play, and you can't do anything with out connecting to a special server.
Now in it's second generation, QuakeWorld has been sculpted by countless hours of user feedback and tweaking to provide the best multi-player experience that can be had from the equipment the average gamer will have. All that is required to use QuakeWorld is registered Quake, and one of the installer files off this site.