Tension, shape, and use.
The GU10 operates at 230V and is commonly used in recessed spotlights and wall sconces. The GU5.3 (MR16) operates at 12V and requires a transformer or power supply.
Never insert a 230V light bulb into a 12V light socket (or vice versa): this may cause damage and is dangerous.
Always check the markings on the existing lamp holder or have a technician measure it.