Kinect features not one, but two cameras to capture various information about the user. One camera, combined with an infrared emitter (the third "pinhole" on the front), provide the 3D information necessary to create the "depth map," while an additional camera captures the standard visible spectrum at 640x480 resolution. With the proper lighting and space, the combination of these cameras allows Kinect to see you in 3D.
Perhaps more surprising than the number of cameras hidden in the sensor is the number of microphones. In addition to three cameras, the device features four downward-facing microphones, three on the right side and one on the left side, integral to the Kinect's voice recognition capabilities. Considering the sheer quantity of power-hungry parts hidden inside the Kinect, it's no surprise that its power demand (12 watts) is so much greater than what a standard USB port can offer (5 watts) -- that's why you'll need a separate power adapter for using it with original Xbox 360 consoles.