Despite the Magic Trackpad's thinness and its support for the same tap-to-click feature as MacBook trackpads, you can also click by pressing down. The trackpad surface itself doesn't depress, though--instead, when you press on the trackpad, the force from the surface it's sitting on will depress the two round feet at the trackpad's base. That's what registers a click.
Think the Magic Trackpad could replace your mouse? Jared Newman doesn't, here are his reasons.