Reddit reviews USB Complete: The Developer's Guide (Complete Guides series)

The two data pins don't convey two separate streams of data (movement and clicks, as you presume). In fact, they carry only a positive and a negative version of the same serial data that is sent from receiver to PC or, vice versa, from PC to receiver.

This serial data 'bus' is the conduit that allows for the wireless mouse receiver to identify itself to the computer (allowing the operating system to download any necessary drivers so as to 'understand' the device), and for the mouse to send back information on mouse movement (up / down, left / right, scroll wheel) or button presses (left? right? scroll click? other?)

The actual and detailed mechanics of just the USB interaction would probably fill several textbooks, but suffice it to say, your $25 wireless mouse "just working" when you plug it in is the end-product of probably millions of hours of work (and billions in investment) in computer interfacing, operating systems development, hardware and software engineering, and getting competing interests together long enough to hash out a common framework.