That picture is nice, but somewhat amusingly excludes the most important part of the headset... the bearings.

It looks like it's supposed to be showing a cartridge headset with the bearings pressed in, but that's not so easy for a reader to understand as a cup-and-cone headset, in my opinion, where you actually see the moving parts.

Wikipedia's drawing of a threaded cup-and-cone headset may be easier to understand: