Trackball may refer to either the hardware used to move a mouse (specifically a ball embedded in the mouse which rolls as the mouse is moved) or to the three.js extension TrackballControls, used in web browsers to manipulate 3D objects. It should be used for questions about software specific to a trackball mouse, or questions about the TrackballControls extension.
In terms of computer hardware, a trackball is a physical ball used to capture input for a mouse. It is fairly rare to see a trackball on the bottom of a mouse anymore, but large and well-made ones are commonly used for high precision work or areas where durable hardware is required.
Questions with the trackball tag, in the hardware sense, are usually related to writing software for a trackball, handling its inputs, and other technical questions. Note that actually asking about the trackball itself is probably off topic, since it's related to computer hardware.
In terms of computer software, TrackballControls is an extension to three.js, which is itself used to control 3D objects in a web browser. It allows the camera to move ("track") around the object. It is fairly similar to OrbitControls.
Questions with the trackball tag, in the software sense, are usually related to the use of the extension, including troubleshooting and proper use.
