The type of laser discussed by bigmouth strikes
is what is called a "solid state" laser. There are several other important types of laser
. The common thread between them is that (a) they have the full mirror at one end and the half mirror at the other (b) to a lesser or greater extent, they produce a coherent
(i.e. in-phase with one another), monochromatic
(i.e. single colour) light source.
As you can imagine, a ruby laser isn't exactly cost-effective. Far more common (and cheap) is the semiconductor laser, also known as a diode laser. Diode lasers are closely related to LEDs; in fact at their core, are really just an LED with the mirrors whacked on either end. These are the lasers in your CD-ROM, in those freaking annoying laser pointers; in fiber optic network equipment and so on. They run at low voltages (say around 5 volts, compared with ruby lasers in the hundreds) and they're much much much more compact. But they are not perfect: they are not purely monochromatic or coherent. They're limited in power though.
Another type of laser is the gas laser. These include helium-neon combinations; and they're much the same idea as the solid state ones; but with gas as the core.