A Moving Magnet phono cartridge is comprised of a tiny magnet, located at the end of the cantilever of the stylus that sits between two coils. One coil caters to the left aspect of the music, while the other is for the right - this allows for the stereo sound. This magnet vibrates between the two coils and induces a small electrical current within them in the process. Because the magnet is so small, it needs less tracking/downward force to correctly manoeuvre between the grooves of a vinyl record. A Moving Iron cartridge makes use of essentially the same construction, but swaps the magnet for a tiny piece of iron or other light-weight and ferrous alloy. The iron is lighter and so reduces the necessary tracking force even further.
Moving Magnet is the most common type of phono cartridge, so many integrated amplifiers now feature a Moving Magnet phono stage and many manufacturers, such as Rega and Ortofon design and construct a wide range of MM cartridges with varying levels of quality. While every MM cartridge makes use of this fundamental construction, the way that this is implemented within a cartridge varies as do the materials used, allowing for differing qualities and a unique sound-signature depending on the brand and model.