Questions:

1) How do lasers work?

Lasers are a kind of light that is different from a light bulb and from the sun.   According to "http://wordnetweb.princeton.edu/perl/webwn?s=laser", 

S: (n) laser, optical maser (an acronym for light amplification by stimulated emission of radiation; an optical device that produces an intense monochromatic beam of coherent light))

2) What do we use them for?

According to "http://www.colorado.edu/physics/2000/lasers/index.html", Lasers are used to cut precise patterns in glass and metal, to reshape corneas to correct poor vision, and to provide intense heat in controlled fusion experiments. But we also use lasers as very precise light sources in supermarket checkout lines, CD players, and to transmit most telephone signals. 

3) Biggest/ Smallest Lasers:

The biggest laser: 

smallest laser: