The key to removing unwanted hair permanently is to destroy targeted hair follicles without damaging the surrounding tissue.
There are normally two ways to achieve this:
One way is by lasers, especially the most frequently used 808nm wavelength laser in hair removal. By selective photothermal decomposition, laser energy is selectively absorbed by colored hair follicles and the temperature of the hair follicles is increased, so as to damage the follicle and inhibit hair growth permanently.
The other way, which is more popular, is by intense pulsed light that we call IPL. It is the same in principle that IPL heats the hair follicles so as to destroy their structure. But it is milder and less painful than lasers.