The primary mouse button is usually the left button or the only button on a single-button mouse and is typically used to activate a user interface control or select text. On most OS, the primary button can be customized (to right or left) in the mouse preferences.
The secondary mouse button is usually the right button, often used to display a context menu. On most OS, the primary button and therefore the secondary button as well can be customized (to right or left) in the mouse preferences.
The X1 button is usually the "back" button when such button exists on the mouse.
The X2 button is usually the "forward" button when such button exists on the mouse.
The auxiliary button is usually the middle button, often combined with a mouse wheel.