What's the difference between a GU10 and MR16?

There are a few differences between them, firstly they have different cap/base types. The GU10 has a twist and lock mechanism while MR16s push straight in.

Another difference is the voltage that they run on. GU10s run on 240v which is exactly the same as your mains electricity, this means you can plug in and go without any hassle.

MR16s run on 12v, this means they require a transformer to change the 240v to 12v. The disadvantage of using a transformer is that they have minimum and maximum wattages, this restricts your options when changing bulbs. If you were switching from a halogen to a LED light bulb you would also require a new transformer because LEDs run on a lower wattage compared to halogens.