Barrel and pincushion distortions are related effects, due to spherical optics. They are most often seen when using a magnifying glass or a convex mirror. The distortions are all but eliminated when modern landscape cameras are focused at infinity, but cameras designed to focus from short distances (<1 meter) to infinity show some image distortion.
When a person puts on new spectacles with more powerful lenses they often notice barrel distortion. It takes several days for the brain to learn to reinterpret the image and to straighten the view of the sides of a door-way. Taking the glasses off then gives pincushion distortion. People who normally wear spectacles can see the distortion if they look for it; in normal viewing they do not notice it.
Barrel and pincushion distortion can be corrected in Photoshop by using the spherize function in the filters index. Recently (2005) a specific lens distortion correction function has been added to Photoshop CS2. The new lens correction function allows pincushion and barrel distortion to be removed over an entire image.