A trackball is an input device which comprises of a exposed protruding ball held by a socket having sensors for detecting the ball's rotation.
There are usually one or two buttons provided with a trackball that have the same capability as click buttons on a mouse. Unlike other input devices which needs movement like the mouse, a trackball is stationary and does not require much space. It can also work on most surfaces which is a great advantage compared to other input devices.
Techopedia explains Trackball
While working with a trackball, the arm and wrist motion are less compared to working with a mouse. The physical strain is less as well. Controlling of the trackball is done using thumbs, fingers or the palm of the hand. As precision control is more pronounced in case of a trackball, it's more popular for certain applications like gaming.
Advantages of using a trackball are:
Less work surface is required for trackball to function.
As it's stationary, the trackball allows continuous and fast scrolling and does not require repositioning.
Precision control is more the case of the trackball. Ergonomic advantages are more pronounced trackballs.
Unlike other input devices, trackballs require only minimal cleaning
Disadvantages of using a trackball are:
Compared to mouse, trackballs are physically larger.
As they are little more expensive, the selection is not as broad as other input devices.