Why do some printed documents have those target-like lines in their corners?
Can't think of any concrete examples right now so I just made this:
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They are trim marks used as a guides to cut the paper.
Sometimes, they are double. One set indicates the limit of the bleed and the other indicates where the paper should be cut. The bleed marks are only for reference and not necessary.
If you prepare a layout print-ready, it's usually better to add the trim marks; it clearly indicates what size is you file.
BUT they are also used on offset printing to help verify the registration of all the plates and make sure they are well aligned. That's why often you won't only these trim marks but you'll also see a little wheel on two or each sides of your document.
They're printed using the "registration" color that contains 100% cyan, 100% magenta, 100% yellow and 100% black; this way they appear on every plate!
Image source: trim marks - extrabolddesign.com, registration mark - all-o-kino.ru
These are crop marks, they indicate how the paper is to be cut after printing. This ensures amongst other things that the color reaches all the way to the edge of your paper*. But you could use crop marks to make custom sized pages, mark your signatures etc.
* Printers in general can't print over page edges, they do on the other hand cut things into size after printing.