A Dutch oven is nothing more than a large cooking pot such as this, that can be used on any stove top. There are also types that are used by campers over hot coals or fires, but for the modern city boy like me, this is what it is.
When I hear 'dutch oven' I always think of the large metal pan and lid my mother has. It is about 5 inches deep and oval in shape. I would think it was about 15 to 18 inches long, large enough to give a baby a bath in. The lid was roughly the same shape and size, but with a handle on top rather than two on the ends.
If I recall correctly it was black with white flecks.
Tweety is right though, generally its a pot with a lid that can go from stove to oven.