Each laser is designed to focus light to a very specific area of the skin - the melanin of the hair. As a certified laser tech, you'll press the laser device onto the skin area of the patient which will start the cycle. The laser light passes through the skins, and affects the melanin (dark pigment) of the hair follicle. The laser emits an intense heat that destroys the follicle thus preventing and hair from growing. Amazingly the surrounding tissue remains unaffected. Just as when you wear dark clothes on a sunny day and you heat up more than with a light or white clothing, the laser works in just the same way on the skin of the patient.
Lasers come in different designs - alexandrite, ruby, diode and ND:YAG. Some treatment centers even offer non-laser light options, even though laser hair removal is very safe on its own. Laser hair removal is nothing new - the technology has been around for quite a while, since the sixties, although it didn't become really safe and mainstream until the nineties. This is because whenever a new device was designed it wasn't well-tested. So there was very little actual permanent hair removal for the patient. Years later in the nineties, the research improved the efficiency and safety of the lasers to bring them to market in a mainstream fashion. Because of the high demand for this treatment, many schools for laser hair removal began to appear. All the laser devices for removing hair are now FDA approved and very safe. The oldest device is the ruby laser. It's only safe on very light skin so it's not widely used because of its lack of treatment diversity.
The other three classes of alexandrite, diode and ND:YAG are the most commonly used today. If you have light skin and dark hair, the alexandrite laser is the best choice because it works well with this contrast. The diode laser is the most flexible and is great for anything with lighter all the way to semi-dark skin tones - also know as Fitzpatrick skin classes I to IV. Finally, if one has very dark skin, the ND:YAG laser device works best on these patients. While all these devices are FDA approved and safe, it is of course only true when done in the hands of a well-trained professional who has attend one of many laser hair removal schools. So it's important to get great training if you're interested in becoming a tech, or find a well-trained doctor or technician if you are a patient. Regardless, laser hair removal is a very safe technology and most likely you'll be in good hands when having the procedure done.