This one is complex. I think that there are articles on wikipedia which can be used for good sources of background knowledge. As the other post indicated quite accurately, there are some articles which are fairly dreadful. The links at the bottom of each article might be where the real research element begins. Exploring these links and the subsequent links which follow can help establish reliability and validity in using wikipedia. I am of the mindset that wikipedia should not be used as a sole source of research. The validity in it comes from researching the links present as the cross chcking of sources can enhance reliability in the research and data collection process.
Reliability refers to how many times you can get the same result when you research an issue. So, as you are searching on Wikipedia, you can find the same information a number of times and count on it to be reliable if you approach it from different ways and keep getting the same results. For example, if you are researching the Russian Revolution of 1917 and you continue coming up with the same storyline for Lenin's death when you use these different search terms: Lenin, Russian communism leaders, Stalin's rise to power... then you are likely on the right path.
Validity refers to the degree of truth something contains. Generally, educators tend to agree that about 90% of Wikipedia is indeed true at any given time. This is why they don't trust it. Educators need the best information all the time. When teaching research, they generally steer students away.
There are some entries on Wikipedia that are excellent sources of information. Others, not so much. Since Wikipedia is a user-generated encyclopedia, it is only as good as its contributors. While they have taken steps in recent months to verify information that is posted there, it is still vulnerable to editing.
A good rule of thumb is to get more than one source for the information you find there. See if there are sources provided with each entry and follow them to see where the information may have come from.
Also, since Wikipedia pays the search engines to be the first entry to show up on the search results, they tend to get overused instead of more legitimate sources of information.
Wikipedia tries to ensure the quality of the information it provides using collective supervision of many different people who may or may not be experts. This results in fairly comprehensive coverage of subjects and generally correct information. However in absence of a facility to ascertain the credentials of contributors responsible for posting any information on the Wikipedia, it becomes difficult to assess the reliability of specific articles posted. I have found Wikipedia most useful in getting a good overview of very wide range of subject. This overview then becomes a basic resource for searching for other material on the subject.
Unforunately, Wikopedia provides neither validity or reliability in any of its answers. Validity and Reliability relate to the "testing" of material using scientific methodology. Wikopedia usually consists of "best guess" information which is submitted and seldom verified for truth in content.
While it provides some useful information, "fact-checking" is really necessary as well as "source verification" before using Wikopedia as an information source