handle protruding from a surface

A knob is a round handle that can be turned or pulled. A doorknob is a round handle that is turned to open a door. A cabinet knob is a small handle that is pulled to open a cabinet door or drawer. A control knob can turn a lamp on and off or make the volume on a radio go up or down.

Control knobs to change the hot and warm air in a hotel room

A "knob" can also be any small, round thing that sticks out.