From 1999 to 2003 there was a website called Nupedia founded by one Jimmy Wales. The idea was that Nupedia was to be an encyclopedia sort of website full of information upkept and udated by people from all over the world with knowledge on the speceific topics each article was about. The issue was that the appeal process was too slow and too complicated. Only 21 articles were accepted in its first year and the numbers were not increasing very quickly. Nupedias Editor-in-Chief, Lawrence Sanger while still working on Nupedia began a pet project called Wikipedia. Wikipedia aimed to be a similar thing to Nupedia however instead of a complicated peer review process remeniscent to an academic or literary journal, the information upkept on Wikipedia was done so by users in the web browser. This is what a wiki is. A bunch of information updated and in theory kept accurate by a large user group. In theory this keeps information as accurate as possinle if everyone is operating in good faith. There are rather interesting worlds of people who upkeep all sorts of somewhat reputable wikis. The website Wikipedia used to be much more lax than it is today which means that just a few persistent and flagrant bad actors can affect the reputability of the site which is why you may have been warned against using wikipedia for any sources in school. Now this idea of a wiki is what Wikia (now Fandom) used to try to emulate. They ran wikis like subreddits and so users can just begin their own wikis. As a result there may be multiple wikis for something and a whole other host of things. The largest issue with Fandom, which I think started to get really egregious when they made the switch to being Fandom from Wikia was the entire near lack of moderation. I think the only instance of Fandom sites being moderated I have ever heard of was when on a character's page on the Sk8 Infinity wiki were thirsting in the comments too hard and so the comments were simply disabled. But like that was it. Wikis run by Fandom have just about no moderation, unlike the popular Wikipedia which is a little less easy to vandalize nowadays. Wikipedia vandalism is my favorite thing in the world tbh coz its often super cute and interesting or really weird and insane. Either way its very entertaining. on top of Fandom wikis being full of badly moderated content there are all of the things about usability and ad clutter that people have been complaining about here and all over the internet for a while now.