Most phones have a front and a rear camera sensor, some have a dual camera on the front and some have a dual or triple camera on the rear.
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 (megapixels) – 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). For example, the camera lenses may have different apertures that allow different amounts of light onto the sensor, using one sensor for daytime and another for low light (night time) pictures.
Some phones include a telephoto lens to allow optical zoom (magnification using physical optics rather than software). For example, the camera lenses may have different focal lengths – a fixed wide angle lens and a fixed telephoto lens. The focal length is the distance between the lens and the image sensor. Typical examples are 2x optical zoom where a different lens is used for the 2x optical zoom. Some phones use software to provide a hybrid zoom, which essentially uses software to combine the image from 2 lenses to provide a mixed digital and optical zoom.