Wikia is a series of wikis (if you don't know what that is, see bottom) that are more in-depth about stuff on Wikipedia. For example, you can find some of the more basic Narnia facts on Wikipedia - there are approximately 110 pages. As for narnia.wikia.com, there are 610 pages. And Wikia doesn’t just have information on books and movies - it also has information on non-fictional subjects too. My favorite hub (category for wiki) is Creative - it has stuff from Alternate History to Con(structed)Worlds. So get out there and start using Wikipedia - and don't forget about Wikia as well.
Merriam Webster's definition of "wiki": a Web site that allows visitors to make changes, contributions, or corrections.