Crop marks indicate where the outer edges of the page will finally be trimmed during finishing.
Bleed is a term which refers to items that go beyond the page trim edge (bleed off). Usually 3mm bleed is added. For example on a single A4 with a full page image (bleeds all 4 edges) the image with bleed added would need to be 216mm x 303mm. When the job is finally guillotined this bleed ensures that the image reaches the trim edges and avoids any white gaps. Bleed and crop marks should be added when creating a pdf file.