How do we know if a site like Wikipedia is reliable? How can we trust the information we are researching? Why?

The answer is that you don't know. At any given time, a Wikipedia article may present you with one of the best online sources on a particular subject, one of the very worst and most misleading, or anything in between. You never really know. Even the much-touted references that appear in Wikipedia articles can be fake, or genuine references that do not actually support the "fact" stated in the article. Due to its largely open and instant editing policy, any troll, vandal, fanboi, spammer, propagandist, revenge-seeker, political hack, conspiracy theorist, tinfoil-hatted nutter, narcissist, power-tripper or libeler can and do add their "improvements" to Wikipedia. Without doing independent research of your own, you can never know what you are getting. The reasons cited above are quite sufficient cause for any responsible educator to forbid student use of Wikipedia as a source, and likely are the reasons so many professors ban it. Still. this merely scratches the surface of why you should not rely upon or contribute to Wikipedia. Click on the second and third links below to read about many more reasons. Unless you are in an advanced class studying subjects like online sociopathy and dysfunction, or the corruption of online resources, there is no legitimate reason to be using Wikipedia as a source in school.

Wikipedia is a very reliable source, and most of its content is true. If somebody vandalizes it, the vandals will get a warning and the vandalism will be removed almost immediately, so most of the information we find will be true.

while you simply cannot trust all the info you find on wikipedia, often the external links and references that you find at the bottom of the page are extremely useful and reliable. especially when you need a source to quote from like in a history or scientific subject where you need to back everything up.

Wikipedia is good for getting a general (and sometimes more) idea of the information out there about your topic. But since Wikipedia can be edited by anyone with an Internet connection, that's probably why your professor wont' allow it as a source. You can trust the Wiki information by cross-referencing with other reliable sources (e.g., encyclopedia, books, magazines, etc.)