Parallax is an effect in photography where the image seen in the viewfinder is not framed the same as the image seen through the lens, because the viewfinder is in a slightly different position to the lens. Also parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight. This effect is most noticeable with subjects close to the camera, and becomes relatively insignificant at longer distances. All cameras with separate viewfinder and taking lens suffer to some degree from parallax error. Cameras which view the scene through the taking lens, i.e. SLRs, don't have parallax problems.
Some viewfinders offer "parallax correction"; this may be some method of shifting the image in the viewfinder, or markings to allow the photographer to frame their shot normally, then shift across to the correction marks to compensate for the parallax.