What makes a laser different from an ordinary light?
Three properties, all following from the fact that laser light is produced by a coordinated process rather than a chaotic one: it is a single colour, the waves are in step, and the beam barely spreads.
How ordinary light is made. In a bulb or an LED, countless atoms emit photons independently and at random moments, in many directions and — in the case of a filament — across many wavelengths. The result is incoherent: a jumble of waves with no fixed relationship.
What "laser" actually describes. The word is an acronym for light amplification by stimulated emission of radiation, and stimulated emission is the whole trick. Einstein predicted it in 1917: when a photon passes an already-excited atom, it can trigger that atom to emit a second photon that is identical — same wavelength, same direction, same phase. One photon becomes two indistinguishable ones, then four, and the process cascades.
What a laser needs to sustain it:
A gain medium — gas, crystal, semiconductor or dye — containing atoms that can be excited.
Pumping, an energy source producing a population inversion, the unusual state in which more atoms are excited than not. Without it, absorption outweighs stimulated emission and nothing amplifies.
A resonant cavity, mirrors at each end bouncing light back through the medium repeatedly. One mirror is partly transparent, and what escapes is the beam.
The three consequences:
Monochromatic — effectively one wavelength, because stimulated emission copies the original.
Coherent — waves in step, which is what makes interferometry, holography and optical communications possible.
Collimated — barely spreading, so energy stays concentrated over distance. A laser bounced off retroreflectors left on the Moon still returns a detectable signal.
What follows practically: lasers read discs and fibre-optic signals, cut metal, perform surgery, measure distance, cool atoms to near absolute zero, and detected gravitational waves. Eye safety is the real hazard — the collimation that makes them useful also lets the eye focus a beam to a damaging point.