Unless otherwise assigned the left button on a three button mouse: Used to select information; e.g.
selecting a piece of text,
selecting a menu item or
selecting a new active window.
The red button is considered to be the primary mouse button, usually it is the left mouse button. (On a single-button Mac mouse, it is the single button.)
If you click with the red button on the desktop you get a World menu.
See FAQ: Mouse Buttons for more info on mappings.
