How is the undo tree used in Vim?

Question

This answer says:

Vim\'s undo/redo system is unbeatable. Type something, undo, type something else, and you can still get back the first thing you typed

Answer 1

A lot of this is summed up here:

http://vim.wikia.com/wiki/Using_undo_branches

Answer 2

If you're using vim, you can navigate through the undo tree using:

• u: (undo) move back in the undo tree
• Ctrl+R: (redo) move forward in the undo tree

Other ways of bringing document back or forward in time:

• :earlier 15m: move back in time 15 minutes
• :later 15m: move front in time 15 minutes