It's an ATS stem (adjustable threadless system). The idea is to make adjusting the stem height along the steerer tube as easy as loosening the stem bolt and sliding the stem up or down what is essentially a giant spacer with the "rail" on it, and then tightening it back up. The headset adjustment stays put during these adjustments. It's not my favorite setup, and the stem can be replaced with a conventional one if you want. It does serve the purpose of making it a little quicker and easier to adjust handlebar height without messing with a spacer stack. I just think it's one of those solutions looking for a problem sort of things.

Not sure if other bike brands equip new bikes with those or not, but I'm pretty sure they can be purchased as an aftermarket item.