Google Docs is an online documentation tool.
Google Docs is ideal for collaborative writing, because it's easily accessible. To set up a Google Docs, you need a Google account, but to comment or edit someone else's (public) file, you do not need to have an account. In Google Docs, you can collaborate real-time, because edits and comments are instantly visible for everyone working in that document. Apart from that, Google Docs allows you to see which revisions were made by whom. Google Docs is a bit limited in terms of design and other text-editing features, especially compared to Microsoft Word.