It is usually a database of in game items and quests. They are generally generated from user mods then get exported to an external site. The external site allows users to search and find things. This generally helps the user get things that he would not be able to get on his own.
Web based talent calculators allow you to find the best way to spent talent points before you spend them. Some games do not allow you to change a spent point. Other make it cost you in game gold. The ability to pre select talents helps prevent errors when making your final choice.
This is usually an item that is not released by the game developers but it still adds value to the game. These range from UI changes to tracking systems to task automation. Damage meters, threat meters, healing stats, raid management, maps, quest and item tool tips, gear changers, and friend trackers are a sample of the types of mods that people like. Some sites are dedicated to hosting mods that the users create.
A game census helps track the population of realms and guild sizes. Some even track players as they change from one guild to the next or the rate that they level.
Profiles are usually a web display of a game character. It can show talent builds, gear, and other honors.
They are the evolution of the in game damage meters and stat trackers into an online historic tracking tool. You get the ability to drill down into the details after the fight and see what's really happening.
Sites that dedicate areas to news related to the game. Some are automated scanners that catch every game developer post. Others dig into patch notes and test servers to give users an idea of what's changing in the game.