Wikipedia is a collaboratively developed, free content encyclopedia. It is very similar to The Elder Scrolls Wiki, although it is a general knowledge encyclopedia, rather than being specific to one topic (as this wiki is The Elder Scrolls specific). Because of this, it is not always particularly friendly to in-depth knowledge specific to a given fictional universe.
Wikipedia could be viewed as a "mother project" of sorts to TESWiki—while there is no official relationship, it is doubtful that this site would ever have existed without the inspiration of Wikipedia. Wikipedia is a wiki that has general information on anything legitimate; in addition, there are also smaller wikis, like TESWiki, that are devoted to specific topics.
It should also be noted that TESWiki was created entirely independently of The Wikimedia Foundation and of any events occurring on Wikipedia, though many The Elder Scrolls Wikians are also Wikipedians, and much information is shared between the two projects. Some TESWikians have even started phasing out of The Elder Scrolls-related activities on Wikipedia to concentrate fully, or at least more, on TESWiki.