The iPhone X features 12MP dual-camera setup, one camera features an f/1.8 aperture wide-angle lens and the a second f/2.4 aperture telephoto lens. This makes the iPhone X more versatile for photography. I compare it to carrying a DSLR camera with two interchangeable prime lenses, what I used to do in the past. The iPhone X rear cameras provide users with the option two choose which camera to shoot with. You can decide to shoot with the wide-angle camera for landscape shots or switch to the telephoto one for portraits or close-up shots. Many phone manufacturers, including Apple, refers to it as "Optical Zoom". Keep in mind that this isn't an optical zoom lens camera, you don't get to zoom through the range from wide to tele, just switch between the two.
The other great news is that both cameras are equipped with an Optical Image Stabilization (OIS) system, what Apple calls "Dual Optical Image Stabilization". This system ensures that you can sharp images and stable videos, as well as help promote better low light capabilities. The reason for that is because the camera an shoot at slower shutter speeds, while the OIS ensures that the image remains sharp when shooting handheld. The OIS becomes even more important for the telephoto lens. The higher the focal length, the more important the OIS becomes. This is because the recommended shutter speed for getting a sharp image is 1/focal length of the lens. With a higher focal length, a faster shutter speed is necessary to maintain a sharp image. In daylight it might not be a problem, but when it starts getting darker, it's necessary to shoot as slower shutter speed to maintain a good exposure, and this where the OIS comes into play and ensures sharp image even when shooting with shutter speeds below the recommended by the "Shutter speed rule of thumb".
The dual-camera on the Apple iPhone X comes with many enhancements, including a Hybrid IR filter, improved Bayer color filter, auto HDR, Portrait Lighting (currently in beta), and many more features. The dual rear camera also supports the new Portrait mode and Portrait Lighting, both help iPhone X owners to capture stunning portrait shots and help you enhance them so they look like they were taken in a studio by a professional photographer. You can achieve beautiful defocused Bokeh effect and apply really stunning lighting effects to really make your portrait shots stand out from the rest.
Here is a video by Photos in Color YouTube user that talks about those exact features.
Having a dual camera is great, but with great complimentary software and dedicated hardware you can achieve really epic results. Just so you know, those five available lighting effects (Apple might add more in the future) happen in real-time not in post-processing after the image is taken. The benefit of this is that you can see the results live on the screen. The IR scanner created a depth map of the face and then knows how to apply shadows and other lighting effects so they appear more natural and not artificial.
All in all, the dual-camera of the iPhone X and the complimentary software really makes this phone stand out from the rest. Apple did take the iPhone X to the next level in that aspect, although those features are also available in the iPhone 8 Plus as well.
The main downside of this setup however is that the telephoto lens is slower (f/2.4 vs f/1.8) compared to the wide-angle. So shooting with the wide-angle will be more appropriate for night shots. That being said, this dual-camera setup is better than the iPhone 8 Plus that comes with a slower f/2.8 aperture lens for the telephoto camera.