High Intensity Discharge or HID that is the family of lamps that Metal Halides fall into. To operate them safely they do require special fixtures and fittings due to the high pressure and heat in which they operate under. Because Metal Halides have such a vast spectrum they are a very popular choice for; home growers, outdoor lighting at football stadiums and other sporting events and a very popular choice for those who own a home aquarium, in fact they are a must for anyone with a marine fish tank set-up.
Without getting too technical a Metal Halide lamp is able to produce such an intensive light (compared to its size) by making an electric arc, this arc is mixed with gases and rare earth metal salts and the result is an intense light with a vast colour spectrum, the spectrum dependant on the mixture.
The 125w Metal Halide is becoming a popular choice for those who are trying to think greener and for company owners who are looking to make savings switching down from the more popular 150w halides. I have seen some aquarium keepers stating that they use the 125w version although it is quite common knowledge that 250w halides are far better and a must for anyone keeping corals.
For those that enjoy growing indoor plants halides are loved and hated. They are loved because of the intensity of the light and hated because of the heat which they give off and their requirement for a ballast to function.