Basically, standard ultrasound imaging only produces a 2-dimensional image as the sound waves used immediately bounce back to the scanning unit. The time it takes for the sound waves to reach the scanner (technically, transducer), along with some more technical details, determines how the resulting image is colored. Any form of ultrasound imaging is not going to look perfect, as the baby's movements can cause artifacts to appear in the image. Also, the high frequency of the ultrasound used limits how clear the image can be, but it's a compromise: low-frequency ultrasound can not penetrate human tissue as effectively.

A 3D/4D (the 4D is only different in that the pictures of the fetus are made very rapidly, giving the impression of live video [it's called this, because, as you might have heard, time is considered to be the fourth dimension]) ultrasound, on the other hand, scatters sound waves throughout the womb and uses much more sophisticated and complicated methods to produce an image.

Comparison:
Standard ultrasound (first image):
Depending on the stage of fetal development and sheer luck, some images may come out much more clearly.

3D Ultrasound (second image):

As you can see, this image is far more detailed. As is the case in this example, 3D ultrasounds are also usually artificially made flesh-colored, probably to give the parent(s) observing more of a sense of familiarity with their unborn child.
4D Ultrasound (video)

As you can see, the picture is a definite improvement. But if you're looking for a family photo to treasure for decades, I'd recommend waiting until after childbirth.

As a side note, there's no real diagnostic reasons I'm aware of that make use of the 3D/4D ultrasound; most conditions would either be caught by a standard sonogram or by completely unrelated methods.
Source(s):
First image from Wikipedia:
Second from: http://www.yourultrasound.com/determining-baby-gender-with-3d-and-4d-ultras...
Video from: http://www.youtube.com/watch?v=GmzUw05bbD0
Additional information:
General info - http://en.wikipedia.org/wiki/3D_Ultrasound and http://en.wikipedia.org/wiki/Medical_ultrasound
http://www.4d-ultrasoundscan.co.uk/articles.htm#Bookmark4Dand3DUltrasoundSc...

Providers of 4D ultrasounds: http://www.4dpeek.com/

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