Crop marks, also known as trim marks, are lines printed in the corners of your publication’s sheet or sheets of paper to show the printer where to trim the paper. They are used by commercial printers for creating bleeds where an image or color on the page needs to extend all the way to the edge of the paper. Printers generally can’t actually print to the very edge of the paper, so instead they print on a larger sheet of paper and then trim it down to the correct size, and crop marks are used to define where to trim. So, to print crop marks, you must print on a paper size that is larger than the page size you have set for your publication.
When would this come up? You might be creating a banner and printing it on your desktop printer using letter-size or legal-size paper. Publisher will print the banner across several sheets of paper that you need to cut and paste or tape together. Crop marks make this process much easier. Or you might want to create bleeds in your publication. Printing the crop marks makes it much easier to then trim the sheet back to create the bleed.
Publisher can automatically show and print crop marks. In Publisher 2003 and 2007, you enable crop marks in the Advanced Printer Setup dialog box:
In Publisher 2010, the dialog box is called Advanced Output Settings in the Printer settings, but it remains essentially the same: