It is most visible in images that have straight lines in them, as these lines appear to be bowed outward.
Barrel lens distortion occurs because the magnification of the image decreases the farther the object is from the optical axis of the lens. So, the portions of the image that are at the edges of the frame are more susceptible to barrel lens distortion with wide-angle lenses.
Some lenses, such as fisheye lenses, actually attempt to take advantage of lens barrel distortion by creating a photo that's purposefully curved.
Fixing Barrel Lens Distortion
Barrel distortion can be corrected extremely easily in modern editing programs such as Photoshop, which contains a "lens distortion" correction filter. Free editing programs also contain solutions to the problem.
As distortion is caused by the effects of perspective on the lens, the only way to correct for barrel lens distortion in camera is to use a special "tilt and shift" lens, which is designed for architectural purposes. However, these lenses are very expensive, and only really make sense if you specialize in this field!
If you cannot prevent barrel lens distortion with a special lens or if you do not want to have to perform a lot of editing on your photos after they are shot, you can try to minimize the impact of barrel lens distortion by following taking care as you shoot the photos. For example, try to avoid shooting buildings or other objects that have clean, straight lines, where the barrel distortion will be obvious. Keep any straight lines in the image as close to the center of the lens as possible. As you shoot an object, shoot several photos of the same object, using a variety of zoom lens magnification levels, as the distortion likely will be less obvious at one zoom level versus another.