Barrel lens distortion

So, you just bought your semi-pro camera, and it comes with the basic 18-55mm lens kit. If you're a newbie like me, it'll take some time until you notice that some photos look weird and distorted when you shoot at 18mm, which doesn't happen when you shoot at 55mm. Maybe you never noticed this is the lens fault, and you thought it was you (or your subject) that was ugly (lol).

It turns out that wide-angle lenses, specially zoom wide-angle lenses, suffer from this barrel effect. It's totally normal and it's even exactly what you want when you shoot with fish-eye lenses, which take advantage of this distortion to make the exposure look curved.

Because it's caused by the effects of perspective on the lens, you cannot prevent this distortion unless you buy a very expensive tilt-shift lens. But you can correct it using image editing software like Adobe Photoshop or GIMP (check out this tutorial). You might also avoid shooting things with straight lines when at 18mm, or just try to keep them at the center. When photographing people, avoid 18mm because they'll just look ugly.

Source: http://cameras.about.com/od/technologies/a/What-Is-Barrel-Lens-Distortion.htm