One of Apple's standard mice, it replaced the infamous Apple USB Mouse, which most everyone unaffectionately dubbed the "hockey puck."
It features optical tracking with an Apple-designed system that has twice the depth-of-field, a 10% higher (better) acceleration threshold, 15% higher (better) velocity threshold, and a 30 degree angle prism for more accurate performance than Microsoft's IntelliEye system, the previous optical tracking champ.
It also has a buttonless clicking surface with adjustable tension, for far superior ergonomics and ease of use. The mouse has an elongated shape that cures the hockey puck's tendency to accidentally be turned sideways, and, like the Apple Pro Keyboard, it's clear, so coordinates with a computer of any color.
The Apple Pro Mouse was introduced on July 19, 2000. It shipped with every new desktop Mac (i.e., the iMac and G4 Cube), and will work on any operating system that supports USB mice (including Linux) without any additonal drivers.
The Apple Pro Mouse was eventually replaced by the Apple Mighty Mouse.