At its core, Google Docs is really a wiki tool. It's essentially a web page that anyone with access can edit. What makes it different from other wikis are the more powerful word processing features, as well as the ability to save your documents to multiple types of formats.
To make sure you don't turn a Google Doc into a wiki that the entire world can edit, it's important to make sure you use the right sharing options. Wired wrote about it this past week (Google Docs Design Flaw May Fool You Into Making Your Docs Editable by Anyone), and I figured it was worth an explanation over here. Head over there for the full article, but the gist is this: when you share a Google Doc with someone, make sure you select the right options so that you don't grant "edit" access to the whole world. The "Sharing" interface could be a little bit confusing, leading users to unintentionally make their documents visible -- and editable -- by anyone, just like Wikipedia.
The Wired folks say (as do I) that this may not be news to anyone -- but in the event someone reading this blog may get confused when choosing privacy options, we've done our part in passing the message on.