As an electro-magnetic 'light wave' with a wavelength of 10,600 nm, a laser beam includes energy that is (partially) absorbed by the substance. The photons, or 'light particles,' transmit their energy to the atomic or molecular structure of the substance, causing it to heat up. If the laser's energy is high enough and the laser is allowed to function for a long enough period of time, the temperature necessary to vaporize the substance develops on the material.
As an example:
Material 1 melts at around 140°C, whereas material 2 melts at 230°C — with 200 W laser power, one may thus cut material 1 faster than material 2, because the energy does not have to be applied for as long to material 1 as it does to material