What is a headset?

The headset is the pair of bearings on both ends of the head tube of the frame. Installing a headset involves pressing cups into the head tube and a ring onto the fork, which can't be done without expensive tools that align all three to be exactly parallel, which usually requires filing off uneven paint. It's one of the few things I let the bicycle shop do.

See also an article on " Selecting a Headset"