See also the official documentation:
Help Center > Members Manual > Working with wiki pages

Some groups don't enable it, others use it heavily, and everything in between.

It can be used for anything that is revised frequently, as well as things that are fairly static. 

Unlike Files, it doesn't use any of the group's storage space, except for any photos within it.

The settings in your group admin, settings, determine who can view, edit, etc.
Even if members have permission to edit, you have the option on each page to only allow moderators to edit it.  

The online wiki editor is essentially the same as the one that you use to create a new message.

But a feature of a wiki is the ability to easily undo edits. 

To revert an edit:

• Go to the wiki page in question.
• Click on "xxx revisions" at the upper right (where xxx is the number of revisions made.)
• Click on the middle column (reason for edit.)
• Scroll to the bottom of that page and select select Delete This Revision.
• Confirm that you want to delete it.
• Done.

For more information see this page on Table of Contents.

To monitor Wiki activity (creation, edits, etc.), see Monitoring Wiki Activity