This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
There are many different reasons this can happen. Far and away
the most common problem is using the wrong printer driver. BEFORE
you start formatting your document, make sure you have a printer
driver for the printer you'll use for the final draft in your system
folder and have selected that printer in the Chooser. Then choose
Page Setup... from the File menu to let the application know what
sort of output it should try to match the display to.
The second most common problem is font confusion. Make sure
you know exactly which fonts are in your document; and, if you're
printing to a PostScript printer, make sure PostScript versions of
these fonts are available to that printer. On newer printers you
might also be able to use TrueType fonts; but PostScript is still
the standard, especially if you're eventually going to Lino for
camera ready output.
The third most common source of trouble is poor formatting,
especially in Microsoft Word. The Mac is not a typewriter, and
you shouldn't use it as one. Don't use tabs as a substitute for
indentation; don't force a page break with carriage returns; and
NEVER use spaces to position anything. If you're writing a resume
(by far the most common source of formatting problems for Word
users), give serious thought to using the well-formatted resume
template that comes with Word to help you avoid problems with
your final printout.