Shortcut can refer to several different items on Ubuntu systems. The two most common are file links and desktop shortcuts.
File links are connections to other files and folders that can be used for easy access. They can be made in Nautilus by using the "Make Link" command, or through the command line with the
ln command. Links have different names on different systems. On Ubuntu they're called links, on Windows shortcuts and on OS X aliases.
Desktop shortcuts are similar, but provide more functionality. They generally link to an application and can be used as launcher files.
Note that this tag should not be used for shortcut keys. The tag for that is shortcut-keys.