These are robots that have no real useful application. They're build for fun and practice but little else more. Most homebuilt robots would fall in this category.
They can be simple robots that use simple sensors (light sensor, sound, bumper switches) and have simple behavior, but they could as well have complex sensors and extensive programming.
One other category of robots that's very close related to this category are robot build as "tech-demos". For example the Asimo robot.