Google Docs is a cloud based tool for creating and editing documents. Quite a simple purpose, but it has a few tricks up its sleeves, like not crashing out like Microsoft word halfway through your masterpiece. But more seriously, you can have multiple authors working on one document with various privileges, making it an amazing tool for collaboration.

Some teams use Basecamp for managing their project documentation storage and comms. It’s a hub for google docs and other resources in a project, and works (though feels very clunky). It also just seemed to be something that Trello could replace easily with good habits in the team for maintaining it.
Personal experience
I use this daily, and feel google have given us a great way of collaborating on documents freely.
Use cases
When you need to get opinions on documentation, or to work collaboratively.
Cons
It’s in the cloud, which scares some meek souls.