This is a pretty good article on parallax http://en.wikipedia.org/wiki/Parallax
Originally Posted by pstake
Parallax is the issue caused by using optics in slightly different positions for viewing and taking a photo.
With an SLR (or other 'through the lens' system) you use the same optics (ie the lens) for both composing and taking the picture, so the image recorded on the film is exactly the same as the image seen in the viewfinder.
With a rangefinder (and also with cameras like a twin lens reflex) you use separate lenses for each job - so one lens mounted on front of the film for recording the image, and a viewfinder for composing it. Because these two lenses are slightly offset from each other they will see slightly different images. At long shooting distances this makes little difference, as the offset is essentially negated by the subjects being further away. At closer distances though the viewfinder being slightly offset means you are going to end up looking over the shoulder of your subject in the viewfinder.
Some cameras will correct for this and move the frame lines as you focus closer, some won't and will just require you to mentally adjust. After you have shot a few rolls of film and see the results you should find yourself being able to tell when parallax will be an issue, and adjust appropriately.