Refer to the chart, and all will be revealed:
http://sheldonbrown.com/handsup.html
22.2mm is the Inner Diameter of the 1" (25.4mm) steerer tube, were you insert the stem. So, the stem has to be 22.2mm in diameter to fit.
That's pretty much standard, except the French. But no one cares about them any more.
Clamp size is the clamp where you insert the bars into the stem. Different deal.