Watch the short "Wikis in Plain English" that very succinctly (and humorously!) demonstrates what a wiki is.

If you want more background and/or prefer to learn by reading and writing, then explore this link on Wikipedia.

Once you feel comfortable with the concept of wikis, create your own account on pbworks.com and play around with it a bit.

THE BASICS
Think of a wiki as a Word document that is stored online and editable by lots of people (the video above demonstrates this very well, I think).
Therefore, if you want to change the webpage or online document,
Hit the "edit" tab at the top of the page; this basically turns the webpage (that you were viewing -- "View" is the only other tab you have) into an editable Word document.
Type the changes you want to make.
When you are through, hit "Save."
Voila! You've created (or contributed to) the webpage!
For more information, check out the User's Manual on pbwiki.com http://usermanual.pbworks.com/ or look at the section on "The Editor" in particular at http://usermanual.pbworks.com/PBworks-Editor.