The Depth Sensor:
Earlier phones using dual camera setup made use of this type of camera setup, the depth sensor. It is the most basic form of camera setup. In this system, the secondary camera accompanies primary camera to 3D map the area in front of primary camera. As we all know two eyes give us 3D view, this setup works on the same principle.Thus secondary camera measures depth of objects and tells how far the object really is. Using this information the camera can separate foreground from background. This setup gives shallow depth of field view. Phones with this type of camera setup are very rare. HTC One M8 was the first phone to use this camera setup.
The Monochrome Camera:
The monochrome camera is slightly more popular than the depth sensor camera system. In this setup the primary camera is accompanied by the mostly identical secondary camera. The only difference between the two cameras being that secondary camera lacks RGB color filter. Thus it cannot take color photos but there is upside to it since there is no color filter thus there is one less thing blocking the camera, means it can capture more light. Every time you click a picture the camera combines image from both cameras to form one picture. So we get a picture with great detail.
The Wide-Angle Camera:
As the name depicts, the second camera has wide-angle giving crazy wide field of view. This allows user to capture wider area without moving back. This was debuted first on LG G5 last year. The lens have big focal length giving wide field of view but as we know something has to be compromised to achieve something. Thus the clarity of wide angle lens is no where match of primary camera lens. There are some distortions in the image when shot using this lens.
The Telephoto Camera:
This camera setup is most common. In this, the primary camera is accompanied by telephoto camera. Telephoto lens is exact opposite of wide-angle lens, this let user to zoom in rather than zoom out. iPhone 7 came with this camera setup providing 2x zoom, since then many other manufacturers have started using this 2x zoom feature. The main advantage of this system is that it provides 2x lossless zoom thus picture quality is not compromised when user zoom in the subject. Any further zoom-in is done digitally. This is done on top of 2x zoom giving better results.
Another advantage of using telephoto lens is for clicking potraits as they are more flattering to the subject and have less distortion. Manufacturers take one step forward and give background blur using software and the results are extremely good.