By default, each Emacs frame has a menu bar at the top which you can use to perform certain common operations. There's no need to describe them in detail here, as you can more easily see for yourself; also, we may change them and add to them in subsequent Emacs versions.
When you are using a window system, you can use the mouse to choose a
command from the menu bar. On text-only terminals, you can use the menu
bar by typing M-` (tmm-menubar). This enters a mode in
which you can select a menu item from the keyboard. Either type the
initial of the item you want, or use the left and right arrow keys to
choose an item and use RET to finalize the choice.
Each of the operations in the menu bar is bound to an ordinary Emacs command which you can invoke equally well with M-x or with its own key bindings. The menu lists one equivalent key binding (if the command has any) at the right margin. To see the command's name and documentation, type C-h k and then select the menu bar item you are interested in.
You can turn display of menu bars on or off with M-x menu-bar-mode. With no argument, this command toggles Menu Bar mode, a minor mode. With an argument, the command turns Menu Bar mode on if the argument is positive, off if the argument is not positive. You can use the X resource `menuBarLines' to control the initial setting of Menu Bar mode. See section X Resources. Expert users often turn off the menu bar, especially on text-only terminals where this makes one additional line available for text.