A menu is a list of commands presented to an operator by a computer system
A computer using a graphical user interface presents menus with a combination of text and symbols to represent choices. By clicking on one of the symbols or text, the operator is selecting the instruction that the symbol represents. A context menu is a menu in which the choices presented to the operator are automatically modified according to the current context in which the operator is working.
Reference: Wikipedia http://en.wikipedia.org/wiki/Menu_(computing)#Types_of_menus, retreived 2011-04-25
Menus most commonly are implemented as part of a gui, but also text-based CLI's (command line interfaces) are possible. Graphical menus are often mouse operated and communicate state and context to the user, i.e. in a navigation.