Some mice have a “wheel” instead of a third button. You can usually click the wheel to act as either Mouse-2 or Mouse-3, depending on the setup. You can also use the wheel to scroll windows instead of using the scroll bar or keyboard commands. Mouse wheel support only works if the system generates appropriate events; whenever possible, it is turned on by default. To toggle this feature, use M-x mouse-wheel-mode.
The two variables
mouse-wheel-scroll-amount determine where and by how much
buffers are scrolled. The variable
mouse-wheel-progressive-speed determines whether the scroll
speed is linked to how fast you move the wheel.