The handlebar stem is the component which connects the handlebars to the fork. There are two types of handlebar stem; quill and threadless.
Quill stems are inserted into the fork steerer tube and tightened using an expanding nut and bolt system on the lower part which is inserted in the fork. They have a 1" diameter lower with an approx 25 - 26mm clamp around the bars.
Threadless stems clamp around the fork steerer tube which extends above the frame which also keeps the headset in place. They have a 1", 1 1/8" and 1.5" diameter steerer tube clamp while the handlebar clamp is either 25.4 (standard) or 31.8 (oversize)