Most phones have a front and rear camera sensor, some have a dual camera on the front and some have a dual camera on the rear, and some have a dual on the front and the back.
The camera sensor converts the light entering the lens into pixels (colored dots). The measure of the detail of the camera is measured in MP (Mega Pixels) – i.e. millions of pixels stored for the image. The larger the MP of a camera, the more detail of an image can be captured by the camera sensor. However, above (say) 12MP this is less important for most users, and the lens quality and image processing capabilities (e.g. in low light) become more important – as well as the photographer capabilities.
Camera sensors are quite small and typically connected to the phone’s main circuit board with a ribbon cable, which means they can be replaced if broken or badly scratched.
It is interesting to note, that some secondary dual cameras are monochrome (not full image sensors) and are used to provide sharpness of the image, while others are full sensors and are used to improve image quality (e.g. reduce noise in low light).