When producing a document which is to be trimmed with a colour, text or graphic right up to the final edge it is important to add 'bleed' to the outer edges. We normally work to a 3mm bleed around each edge, as shown below..
As you can see the graphic is overrunning the page 3mm from the edge.
The reasons for this are to do with small movements of the paper, mainly from the final cutting on the guillotine after it has already been printed, and ensures that your final print is exactly how you want it and doesn't have a tiny white border round the edge.
Update - 19/12/2011
As an update, i've done a new graphic explaining the bleed, trim lines and safe areas as well, which is below -
Example - Your leaflet is to be A5, which has a finished size of 148mm x 210mm, the supplied artwork should be 154mm x 216mm. So, the final size plus 3mm all the way round.