The difference is in how the lamp works.
Fluorescent are sealed tubes filled with gas and a little mercury. When the electricity is turned on electrons will migrate through the gas from one end of the tube to the other and some of the mercury turns from liquid to gas. As the electrons move through the tube they collide with the gaseous mercury atoms and emit light.
A metal halide uses a arc tube inside the outer bulb. The arc tube is filled with a gas mixture of gas, mercury and MH salts. As the lamp starts the gas mixture vaporizes and like fluorescents emits light. This so called single point lighting emits a much stronger light than fluorescents.
Both types use a phosphor powder coating on the inside of the bulb to determine color. These phosphors get worn out through time and the spectrum will shift... this is why we have to change out our lights so often.