The laser diode is the most used laser type today. It is used in laser printers, CDROM drives, laser pointers (annoying things) and several other devices.

Build up of a normal laser diode:
    Beam : 
       __:__ end facet  
      |  |  |
      |  |  |
  O---|P | N|---O
      |  |  |     laser
      |__|__|     chip
    beam :   end facet
  O---|  |  |---O

The end facets is simply the end of the semiconductor material that works as the mirrors in the laser. Depending on angle the photons hits the facet will or will not be reflected(the farther from 90 degrees angle they come more of them will be reflected).Some of the reflected photons will hit atoms in the junction and stimulate the release of more photons.

nonreflected light
               V \
               |  "`:
               |    :
---------------|----:--------- facet
     / \       |   | :              
    /   \      |   | :                                    
   /     \     |  |   : 
  /       \    |  |   :
 /         \   | |     :
/           \  | |     : 
             ^         ^
             |         |
         Reflected light

In some diodes the surface is specialy coated to only let near 90 degree angled light through, to make a tighter beam. The photodiode's output is used by the driver electronics to measure the laser chip output. This is to prevent overloading the chip. If the chip is feed a to high current, even for only a few miliseconds, the concentrated beam can damage the facets and the PN junction.

The laser chip and photodiode is usualy mounted in a metal package(to conduct away the generated heat) with a transparent top. As the lightangle of this type of diode can be up to about 40 degrees, a so called collimator lens is added if a thin beam with low spreading is wanted.

The laser diode has a long lifetime(if properly regulated), a high efficiency and are small compeared with other laser types. But their small size is also a shortcoming, since their resonance cavity is so small(less than a milimeter where a gas pumped lasers would at least be 10cm). This means that the diode laser's light is less coherent and monochrome. But in many applications the laser diode is more than adequate.

The laser diode is available from small units, with 1mW-10mW effect, to arrays of laser chips with a output of 100W. These arrays are used to drive(pump) Diode Pumped Solid State(DPSS) lasers with several kilowatts output effect.
The laser diodes that are available today is mostly in the infrared to red(670nm) spectrum. But other colours has been developed and are likely to be available soon.