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.