If you are doing schoolwork, or checking someone else's work, or writing an essay, or writing a book,
Phew. Thanks for letting me get that off my chest.
I probably don't have to relate the problems of Wikipedia, which Cecil Adams of the Straight Dope once called "the million-monkeys-with-a-million-keyboards approach." Anyone can edit it, if he's persistent about it---even a non-tech guy like the Washington Post's Gene Weingartner, who once put a bunch of whoppers about himself on his Wikipedia page to see how long it would take for anyone to notice. (In his case, slightly more than a couple of days.)
Some of the entries are terribly written, which is the least of their problems. Many are poorly researched, and the best the editors can do is flag the page and beg the public for help. There is no standardization, not even at the level of IMDb; an article may be long and meticulously documented and researched, while another on a very similar subject may be brief, sloppy, and free of references. It also has no sense of scholarly proportion; the English language Taylor Swift page runs close to 23,000 words, with 544 footnotes (as of this morning); the article on England, about the same length, with 345 footnotes.
Having said all that, I do think there is a place for Wikipedia in research, that being as a possible jumping-off point. The pages that are properly sourced are done well, and can be a good catchall for up-to-date information. But every fact must be checked against the source, and those sources themselves can vary wildly in quality. An article on a dietary mineral may quote the Institute of Medicine and Dr. Spurious McQuack's syndicated crapfest of a TV show.
The people who write and edit Wikipedia do try, but they have neither the resources nor the focus of the World Book, let alone the Encyclopedia Britannica. All I can say is that Wikipedia itself can never be a source, and all its research must be taken with a grain of salt, which Wikipedia tells us was an ingredient in Pliny the Elder's antidote to poison.