IntelliMouse
Alternatively called the Microsoft mouse, wheel mouse, or scroll mouse, the IntelliMouse is an optical mouse brand developed by Microsoft in 1996. It has a wheel between the left and right buttons for scrolling up and down a web page without using the vertical scroll bar. The picture below shows an example of the original Microsoft IntelliMouse.
Due to the wide popularity of this mouse, it has become the standard mouse model used with most computers. Although "IntelliMouse" is a Microsoft trademark, wheel mice (mouse with a wheel) are developed by every mouse manufacturer today.
Other functions of the mouse wheel
Mouse wheel button
All modern mice with a wheel also act as an additional mouse button when pressed. For example, if you hover over a link and press down on the mouse wheel, it opens that link in a new tab (unless changed).
Free-spin wheel
Some more expensive mice may also have a free-spinning wheel, which can be flicked and continue to spin for a few seconds or until stopped. Using this ability, you can scroll through several pages in a few seconds or quickly get to the beginning or end of a document.
Tilt wheel
Some wheel mice can also tilt the wheel left-to-right. This ability is used for horizontal scrolling. However, the tilt wheel can also be made to do any action (e.g., moving forward and back on a browser).