ETTV allows you to connect up to a special server using your PC, and view a game in progress much like a spectator would. But it's more powerful than any normal spectator function - ETTV allows you all the features of a shoutcaster - being able to see the names of players over their heads, seeing a dynamite timer over the dynamite, etc. You can even watch spectators on the server, such as 'cameramen', people that ensure you get the best overview shots of battles, the best 1st person views during 1 on 1's, and those crucial views just before a panzer dishes out some ownage on an unsuspecting enemy group.
Because you're using ETTV rather than just spectating the game, the server can relay the broadcast of the game to other servers - meaning 100's or even 1000's of us can watch world-class teams battling without lagging them out by crowding their server.
Documentation : Wolfwiki.anime.net