Michael, Barrel distortion is associated with wide angle (or minimal zoom) lenses and it causes the images to appear spherical (curved outward). You can notice this when you have straight edge near the side of the image.
De-barrrelizing is the term used for the process of correcting this distortion. You can do this in PS or use one of many other freeware, shareware and commercial programs. This distortion (and the need to correct for it) can be minimized by careful framing of the image. Keeping the horizon in the center of the frame and straight lines away from edges pretty much renders an image that more times than not does not need to be corrected.
Here's an uncorrected image taken with the 10D and Sigma 15 fish:
Here's the corrected version (note you lose some of the image's field of view when correcting it and the image quality around the edges can suffer negative effects if you don't use a process that employs high quality resampling)
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