Laser is basically a type of light that carries only a specific wavelength, as opposed to visible light that carries a range of wavelength. Laser waves are parallel to each other and all waves move temporally in the same phase. A visible light are not parallel and does not move temporally in the same phase. There are many lasers in the market. Nd:Yag laser is named after the laser medium, Neodymium:yytrium-alumunium-garnet.
It comes in 2 wavelengths, 1064 nm and 532 nm. It is a non-ablative laser. This means that there is no break in skin tissue continuity during the treatment. It is designed to target pigment cells. This specific wavelength is absorbed preferentially by pigments (melanin) and the resultant heat will cause generation and propagation of waves that cause damage to the cells containing pigment. These damaged cells will then be cleared from the site resulting in lightening of the pigmented site.
Q-switched Nd:Yag laser are used mainly for pigmentary disorders resulting in darker patch/lesions. It can target both superficial and deeper skin pigmentation. However, not all lesions have good results with Q-switched Nd:Yag laser.