The headset is the component on a bicycle that provides an interface between the bicycle fork and the bicycle frame
itself. The short tube through which the steerer of the fork runs is called the head tube.
A typical headset consists of two cups that are pressed into the top and bottom of the head tube.
Inside the two cups are bearings that provide a low friction contact between the bearing cup and the steerer.
Headsets come in several different diameters, such as 1", 1 1/4", 1 1/8", and 1 1/2" (often referred to as OnePointFive), so a frame's head tube,
a fork's steer tube, and the headset's diameter all have to be compatible.Full Article