By pressing certain key combinations, you can do things that normally need a mouse, trackpad, or other input device.
Command-B: Boldface the selected text, or turn boldfacing on or off.
Command-U: Underline the selected text, or turn underlining on or off.
Command-I: Italicize the selected text, or turn italics on or off.
Command-X: Cut the selected item and copy it to the Clipboard.
Command-C: Copy the selected item to the Clipboard. This also works for files in the Finder.
Command-V: Paste the contents of the Clipboard into the current document or app. This also works for files in the Finder.
Command-Z: Undo the previous command. You can then press Shift-Command-Z to Redo, reversing the undo command. In some apps, you can undo and redo multiple commands.