This is the correct technical name for the individual light receptors on a sensor, though many people call them pixels because each photosite corresponds to a pixel in the final image. Each photoreceptor gathers light (photons) and turns them into an electrical charge (electrons) which can be measured.
The size, resolution and sensor technology used by digital cameras is constantly changing, but there are some basic principles which don't really change that much and will help you decide between one camera type and another.
This is the physical size of the sensor, which is independent of the number of megapixels it has. Bigger sensors capture more light and produce sharper, clearer images with less noise. In fact sensor size is the single most important factor these days in a camera’s picture quality – megapixels are mostly secondary.
The number of pixels captured by the camera’s sensor. Smartphones typically have around 8 megapixels and upwards, while regular digital cameras typically have 16 megapixels or more. Megapixels used to be a good guide to image quality but now sensor size is more important.