Difference between lights?

Whats the difference between hps and metal halide??? Do they both run off the same ballast??? are they just a matter of switching globes or is it a completely different set up?
I been using my Hps for veg and flowering for years now and never really gave it much thought... Maybe i could increase my results!? What effect would it have on the final product? oh and why? something to do with light spectrums i take it...

Anyone help with this question?

They do have different ballast so, you cant put a m/n in a hps ballast. M/H is more of a blue color so it is a good light for veg. They do have conversion bulbs that make the bulb a different spectrum.

metal halide is the spectrum plants use for the growth stage in their lives, you can get ballast that can run both by a switch on them. hps is mainly for flowering/budding 12/12 cycle. it's the color spectrum the plant is expecting then. you can use hps for both.. no problem.. you just get more growth, closer nodes, denser plants with metal halide when you used them for veg.

Oh i see thanks guys!!! Well i better go see my mate at the shop then and see whats availiable!!! hahaha cheers fellas!