I believe the Horizon camera is an example of a slit-scanning camera. During the exposure, the lens assembly rotates from one end of the panoramic field to the other. A narrow slit is used to ensure that only a thin line of film is being exposed at any instant. The result is that the whole image is exposed using the center of the lens, which can form a high-quality image, and you don't get distortion, which is typical on very wide-angle lenses. The scanning method also results in a cylindrical projection, which may be more suitable than the rectilinear or fisheye projections you get from camera lenses on an ordinary camera. (Rectilinear and fisheye lenses can really stretch people in unattractive ways, so slit-scan cameras were great for photographing crowds of people.)

One example of a digital scanning panoramic camera is the Panoscan, which takes medium-format lenses, and scans mechanically to create an image using a trilinear CCD (one column of pixels in each of red, green, and blue) to form an image.