Laser stands for "light amplification by stimulated emission of radiation." All lasers consist of a lasing material, such as dye, crystal, or gas, to create the wavelength; a pump source, such as a flash lamp or electrical current, to increase the energy of the lasing material; and an optical cavity with mirrors to amplify the light. Here's an explanation of each term in the acronym.
Light is the visible product of a laser. The key difference between laser and ordinary light is that laser light all goes in one direction and consists of one wavelength of the electromagnetic spectrum, which includes (in ascending order of energy and wavelength) radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
Amplification Like everything else in the universe, the lasing material is made up of atoms, each of which consists of a nucleus and orbiting electrons. This step, pushing those electrons to a higher energy level using heat, light, or electricity, is the start of the laser process.
Stimulated Impossible to separate from the next term, but done here for the sake of clarity. Technical term denoting the state of the amplified atom. Having been artificially pumped up, the atom is highly unstable, which leads to the next phase of the process.
Emission The return of the amplified atoms to a lower energy state. When this happens, the atoms release energy in the form of photons of light, all of the same wavelength. In a kind of chain reaction, the photons cause the amplification of other atoms and thus the release of more energy.
Radiation In its broadest sense, radiation refers to the emission of any form of energy, not just radioactive particles. In a laser, it means the beam of light produced by the process.