From what i know, digicams have either an aspect ratio of 4:3 or 3:2.
What this means that the 4:3 ratio size will give you an image that is slightly longer (on one side) than the normal print sizes. So, if your camera is on 4:3 aspect ratio, and you would like to print that image straight out to a 4R or 8R size, your print will be cropped. For example, a 4R print dimension is 4"x6". But a 4:3 ratio image translates to 4.5"x6" - meaning you'll get half an inch cropped off.
Usually, what you can do is go to Photoshop and do your own cropping before you send it to the printer.
different destination has different aspect ratio (shape) so a DSLR output simply printed on A4 with the "stretch" option turned on will render your picture horribly out of shape because DSLR has 1.5 aspect ratio whereas A4 (in fact all the A-series papers) has 1.414 shape.