Undo and Redo

When you are in an application like Word, you can reverse your actions by pressing Control + Z.
You can keep pressing Control + Z to undo each bit of typing or formatting that you performed. According to Microsoft, you can undo up to 20 actions, but I think it may be more.
You can also access the list of actions you want to undo by pressing Alt + 3. Arrow down to the point you want to go back to and press Enter.
Note: The Alt + 3 shortcut accesses the Quick Access Toolbar. If you have customised this toolbar, you may need to use a different number.
There is also the Redo shortcut, Control + Y, which will Undo your Undo!
Or Control + Y can be used to repeat your actions, so you can type the same thing over and over.