The formal definition: A device that converts some form of energy (electrical, optical, chemical, etc.) into a narrow beam of light which is monochromatic (single pure color) and coherent (all waves in step with one another).

Monochromatic means that it consist of one single color or wavelength. Even though some lasers can generator more than one wavelength, the light is extreme "pure" and consists of a very narrow spectral range.

Directional means that the beam is very well collimated and travels over long distances with very little spread in diameter.

Coherent means that all individual waves of light are moving precisely together through time and space, or are in phase. The effect of one wave enhances the strength of every other wave, so that the overall effect of coherent light is much greater than if the waves were not in phase.

Because of these properties laser light can be focused to an extremely small spot, which results in a very large power density which produces a very high temperature.

Lasers come in many different types, each with a different power level and wavelength (color). Some are so weak that you cannot feel the beam on your hand (i.e., supermarket scanners), while others might have an invisible beam that can burn a hole through a steel plate (large CO2 laser).