Keyboard shortcuts are typically an alternate means for invoking one or more commands that would otherwise be accessible only through a menu, a pointing device, different levels of a user interface, or via a command console. Keyboard shortcuts generally expedite common operations by reducing input sequences to a few keystrokes, hence the term "shortcut".
Some keyboard shortcuts require the user to press a single key or a sequence of keys one after the other. Other keyboard shortcuts require pressing and holding several keys simultaneously. For simultaneous keyboard shortcuts, one usually first holds down the modifier key(s), then quickly presses and releases the regular (non-modifier) key, and finally releases the modifier key(s).
[Copied from the old keyboard-shortcut wiki]