What is a laser?

I'll start things off by asking the first question myself... what is a laser?

A laser is a light source that exhibits several properties that make it unique compared to other types of light sources (incandescent and fluorescent light bulbs, the sun, etc).

1. Laser light is monochromatic - this means that the light that comes from a laser is a single wavelength (one color). This is why you usually see red or green lasers, for example, and not white lasers. The color white is a combination of many colors.
2. Laser light is coherent - this means that all the light "waves" are organized with respect to each other. Think of this like synchronized swimming, whereas most other light sources are more like the random collection of people swimming down at the local pool.
3. Laser light is directional - ...hence, the laser beam. Whereas, your typical light bulb emits light in all directions.

These properties of lasers make them extremely valuable tools for many applications including metrology (precise measurements), remote sensing, industrial processing, medical procedures, and, of course, the all-important laser light show.