Keyboard Shortcuts are keystrokes used to quickly invoke a command. As well, shortcuts usually (but not always) involve a modifier key such as Ctrl/Cmd, Alt/Opt, or Shift. For example, instead of choosing Select » Deselect from the menus, you could simply press Ctrl/Cmd + D.
Keyboard shortcuts can improve your efficiency, and in some cases, they can even invoke commands that might not otherwise be available (e.g., zooming while a dialog box is open, or inserting a Select forward layer command into an action).
One of Photoshop's strengths is that it makes extensive use of keyboard shortcuts; but, there are so many that it's difficult to remember them all. So, I've created a "cheat sheet" that's organized by menu, as well as alphabetically by key. These "cheat sheets" are available in the following flavours: