Crossed lines placed at the corners of an image or a page to indicate where to trim it are known as crop marks. Crop Marks may be drawn on manually or automatically applied with some desktop publishing software programs.
Crop marks are typically used when printing to a larger sheet of paper than the final trim size of the document, especially when doing bleeds. They indicate where to cut the paper. Similar to crop marks, center marks indicate the center of a spread and usually mean that is where the page is to be folded.