The Menu interface is an interface characterized by the presence of a menu button where commands are selected as opposed to a command line where commands are typed in.
A menu interface provides a very simple way for users to get started with a product and requires less hardware than other techniques. It is often the heart of a system where there are very few hardware keys present. Typically one of the keys (or a item on a touch screen) will be used to bring up a menu of choices. This list of choices will be traversed and a selection made to perform the desired task.
The big advantage of a menu system for the user is that he or she does not need to remember any commands. Instead they only need to recognize what they want from a list that is presented to them. The disadvantage and that a list can get cumbersome if it becomes too long and some of the advantage is lost if a large number of sub menus are used or multiple menus since you can see everything at once requiring a search.
The big advantage for the manufacturer is that a menu system requires only a few keys which saves costs and space over providing a full keyboard. Generally a menu key, 4 direction keys (often a keypad) and a selection key (enter key) is all that is needed.
 Action keys
A menu is easy for a beginner to learn on but if it is the only interface it can be frustrating for a power user since it requires several steps to get to a command. Thus it is usually supplemented with action keys for often needed commands. For an eBook reader the most often needed keys include a next page key and a previous page key so that a single button push is all that is required for these vital functions. It is possible to share tasks by using the direction keys to double up as action keys when the menu is not in use.
Doubling up key usage is a form of context, meaning the key use depends on what you are currently doing or what is currently being displayed. Another form of context is that the menu key itself can display a different list depending on context. For example, while you are reading a book the menu key can offer different choices that are offered when you are browsing a library screen to find a book. Usually there is one menu that is defined as the main menu and there are several context menus. In some cases tapping the menu key twice in a row will select the main menu or it may be a choice on all context menus.
 Main Menu
The main menu normally includes set up items as well as regular commands. Everything a user might need to do or need to know will have to appear somewhere on the menu. Good menu design will make these things easy to find.