You have a report to write on the Vietnam War. No problem, you did your work. You found your references - all six of them.
Wait ... six? You thought you only needed five! Well you'd better think fast, your report's due tomorrow morning and it's already midnight. What do you do?
Well you could try a Google search, but even the best search engine is hit or miss sometimes. No, your best bet is to use an encyclopedia.
What, your edition of Encyclopedia Britanica was published in 1956? Never fear, Wikipedia is here!
Wikipedia is based off of two words, the combination of which is a very accurate description of the site. The first part, "wiki," describes what can be called an "open source" web site - anyone can edit it. This is great, because if it's missing a section, some enterprising individual is bound to add it. If information is found to be incorrect, someone else will fix it. Yes, there's the potential for someone to really screw this up, but so far so good. (I'll let you guess what the "-pedia" part stands for.)
So, if you want to use a free online encyclopedia that gets updated almost in real time, check out Wikipedia. You won't be disappointed.