The Dobsonian Telescope (or Dob) is credited to John Dobson who first started using them in the 1950s in an attempt to bring astronomy to the public. The Dob is essentially just a Newtonian Reflecting Telescope tube mounted on an inexpensive mount (called an alt-azimuth mount) which holds the tube like a cannon, allowing it to easily swivel up-down and left-right. The Dobsonian therefore shares many of the good/bad points of the Reflecting Telescope (click on the link for more information). The mount is usually of wooden construction, and, since the mount consitutes a significant proportion of the cost of a telescope, Dobsonians tend to be much cheaper than other similarly sized telescopes.
One big advantage of the Dobsonian is its ease of use: you can be outside and set up ready for viewing within minutes. The simplicity of the mount means you just point the telescope to where you want to look and that's it, no complicated polar alignment is required (as is the case with equatorial mounts). You will need to keep nudging the telescope to keep the object you're looking at in view as the night sky revolves, but in practice one gets used to that quite quickly.
The compact nature of the Dob's mount means that the Dobsonian is highly portable, and smaller models can easily be taken by car to dark-sky locations.
The low centre of gravity of the Dobsonian means the telescope is very stable and doesn't suffer as much from vibration, unlike some other tripod-mounted telescopes (vibrations cause the image in the eyepiece to dance around and one must wait for them to damp down before viewing - in a good mount vibrations will die away quickly leaving a steady image).
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