The Other Wiki. The wiki that most people are familiar with. The one that isn't us.
Wikipedia is the most famous wiki out there, and is mostly responsible for inspiring the creation of other wikis (although it was not the first). It presents its information as an encyclopedia and focuses mainly on real-life information.
Given Wikipedia's role as a central information source, you can probably gain more info on the "what" of (for example) Star Wars or Star Trek from it than you can from actually watching the show, and that's nice. Here?
Here, you can get a glimmering of why the show is like that.
Here at TV Tropes, we care only about how things apply to fiction. Don't just tell us the facts; tell us the memes, tell us the archetypes, tell us the catchy ideas and symbolic roles that get planted in people's heads. Got the kernel of an idea bouncing around your head? Throw it down here and see what grows. If we're lucky, our neologism for it will catch on.
... Where were we? Oh yeah.
Wikipedia was started in 2001 by Jimmy "Jimbo" Wales. In addition to popularizing wikis with Wikipedia, he also started the website Wikicities (later Wikia, and currently Fandom), which is where you'll find most wikis that aren't Wikipedia. Since Wikipedia is one of the most popular websites ever, it gets thrown around in the media sometimes; see Wiki Tropes. (Small Reference Pools is in full force here.)
Wikipedia also has an entry on us. It also lists us in its directory of alternatives, encouraging people to record their trope knowledge here, instead of there. See the We Are Not Alone Index for tropes that have Wikipedia articles.
Oh, and if you're a frequenter of both TV Tropes and Wikipedia, try not to mix up the two!