The gain medium is pumped with energy from an electrical current. When atoms in the medium absorb the energy, their electrons become excited, and are promoted to higher energy levels. The electrons then de-excite, falling back down to lower energy levels to emit photons, which are the particles or packets of energy that make up light.
|From Mini Physics|
Laser light is coherent - the waves are 'in phase', or the photons are in sync with each other - and monochromatic, as the waves are of one wavelength. It is also very directional, which means it's concentrated and doesn't really scatter. You can't see a laser beam going through the air unless there's smoke or a lot of dust for the laser to reflect off into your eye. A lot of films don't seem to care though...
If you see any clothes with some kind of really detailed cut-out pattern, it was probably achieved by lasers. Who said physics and fashion can't mix?