A laser is essentially a parallel beam of light that is of a certain wavelength. The extensive use of skin lasers in dermatology started actually not that long ago. But we have come a long way from the early days of dermatological lasers.
Ablative Lasers
One of the first dermatological lasers were invented back in the 1990's. And I had the pleasure of operating one of these. It was a Sharplan CO2 laser and this was 1995 or so. The carbon dioxide laser would be fired onto the skin in minute pulses that caused the ablation of anything it can into contact with. But because it was focused into a tiny spot when it hit the skin, one could do really precise work with this laser. The CO2 laser cut, cauterized and and vapourized all kinds of unwanted skin lesions. Things with which it was particularly good at were warts, skin tags, seborrheic keratoses, some moles and even some superficial pigmentation patches.
Selective Photo-thermolysis
So from that early laser, we learnt that you need a target which we call a chromophore, because we are using a laser which is a light source a form of energy to destroy tissues. In the case of the CO2 laser, the chromophore was water (water in the skin cells) as the wavelength of the laser was well absorbed by water. But water being so ubiquitous in the cells in the skin, CO2 lasers were not very selective in their destructive power. We basically called these lasers ablative lasers. Good for cutting through tissues and vaporizing anything in its path.
Then scientists and doctors started playing with different wavelength lasers and discovered that there are other target chromophores in the skin which were selectively destroyed when light of a certain wavelength hit them. Chromophores like red blood cells within blood vessels, and melanin pigments reacted to very different wavelengths. Hence lasers like the Ruby laser, Alexandrite laser, Nd-Yag lasers, V beam, Smoothbean, Cooltouch, Coolglide and many more came about to treat the different chromophores. Refinements in technology lead to the incorporation of cooling to protect the surrounding tissues from collateral heat injury.
An offshoot of this technology was the Intense Pulsed Light (IPL) which used broad spectrum light (meaning many wavelengths in the beam) to target the chromophores. This was in theory a cheaper light source to generate than lasers. You only need an old fashion flashlight to generate the energy to target the chromophores. In practice because of all the surrounding precision cooling and monitoring, it did'nt make that much difference in price. The idea was that when broad spectrum light was shone on skin, the pigment spots, hair and blood vessels would absorb more of the light and self destruct. The surrounding skin which does not absorb so much light would be unharmed. But still benefited from a mild rejuvenative effect.
Fractional Technology
The 3rd advancement in skin laser technology was fractional technology. Fractional anything became the buzzword in around year 2000. Fractional means instead of firing one beam of laser, you was a scanning device to fire a number (usually hundreds) of beam onto an area of skin. The important thing is that these beams will purposely leave a space between adjacent beams. Looking directly onto skin, it would appear like a pixelated areas where laser had been fired interspersed with normal skin. This is the fundamental theory of fractional technologies. It left normal skin surrounding each area where laser had been fired, so that healing time is much faster. This also enable a mild skin tightening effect when the healing occured circumferentially around each of the spots of thermal injury.
Of course other that just lasers, we now also have radiofrequency, plasma, electrocautery, and focused ultrasound as energy sources. But because they are not lasers, I have left them out in the discussion here.