Photons interacting with electrons are either absorbed, lifting the electrons to an excited state or they stimulate the release of energy from electrons which already are in the excited state (stimulated emission). The energy is released in form of a second photon, identical in frequency, polarisation, phase and direction of propagation to the stimulating one.
Inside an active laser medium (gas, crystal) electrons are pumped to a high energy excited state by supply of external energy (e.g. electric discharge). This results in a thermodynamically unstable condition termed population conversion: More electrons are in the excited than in the ground state. Thus the probability of incoming photons to stimulate emission is higher than the probability of being absorbed. This is prerequisite for light amplification, because now each stimulated emission event caused by one photon releases two photons (the stimulating and the stimulated one). Losses by absorption are minimised.
In a laser the active medium is placed between two mirrors such that the light is lead through the area with population conversion again and again (optical resonator) causing ever increasing amplification factors until the system is saturated by reduction of population conversion and other losses. The laser light is coupled out through one of the mirrors, which is slightly permeable for the respective wavelength.