Laser is an acronym for "Light Amplification by Stimulated Emission of Radiation". It uses a quantum mechanical effect, stimulated emission, to generate a very collimated, monochromatic and coherent beam of light.
Common light sources, such as the electric light bulb emit photons in all directions, usually over a wide spectrum of wavelengths. Most light sources are also incoherent, i.e., there is no fixed phase relationship between the photons emitted by the light source.
By contrast, a laser emits photons in a narrow, well-defined beam of light. The light is often near-monochromatic, consisting of a single wavelength or color, is highly coherent and is often polarised. Some types of laser, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds).
Laser light can be highly intense — able to cut steel and other metals. The beam emitted by a laser often has a very small divergence (i.e. it is highly collimated). The beam will eventually spread due to the effect of diffraction but much less so than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile in diameter if shone from the Earth's surface to the Moon.
A laser can also function as an optical amplifier when seeded with light from another source. The amplified signal can be very similar to the input signal in terms wavelength, phase and polarisation; this is particularly important in optical communications.
To understand lasers you should know some definitons such as
Wavelength - The length of the light wave. The shortest distance at which the wave pattern fully repeats itself, usually measured from crest to crest. The wavelength of light in the visible spectrum determines its color. Common units of measurement are the micrometer (micron), the nanometer, and (old unit) the Angstrom unit.
Frequency - The number of times that the wave oscillates per second (The number of periods of oscillations per second). Common units of measurment are Hertz.
Period - the minimum distance between two adjacent points with the same phase in vibration processes. It is measured in seconds.
Power - The rate of energy delivery in a unit of time, expressed
in Watts (Joules per second). Thus: 1 [Watt] = 1 [Joule]/1 [sec].