If you are looking for a more structured information please visit the Versioning article.

What kind of content may have versions?

Any kind. Versioning is switched OFF by default, but you can switch it ON on any kind of content - even folders or workspaces.

How do I see and restore old versions?

On the admin UI you will find a Versions menu item in context menus. There you can review and restore older versions.

How can I delete old versions?

Currently this is not possible, all versions are preserved if versioning is switched ON.

Who can see the versions of my file?

Major versions (like 1.0, 2.0) can be seen by anyone with Open or even Preview permissions. Minor versions (like 1.1D, 2.3R) are visible only to users with OpenMinor permission.