Macintosh computers operate at the typographical standard resolution of 72 dpi. This makes it very easy to convert from pixels to points, because they're the same! This means that if you have the correct monitor size, your screen image will be the same size as what you print. There's 72 points in an inch, there's 72 pixels in an inch. Very nice.
Windows2 computers run at a standard resolution of 96 dpi, and I have no idea why (might be related to the dpi of some early CGA/EGA/VGA screens?). Because of this, objects that are displayed under Windows will appear to be 133% of their printed size. I can't tell you how many times I see students in the lab set their type size to 9 pt on a Windows workstation (appears to be (9 * 3/4) / 72 = 1/6" on screen) and wonder why it looks so tiny when printed (9/72 = 1/8" on paper).
Footnotes, revised 12-2000:
1. The 832x624 is apparently a really uncommon PC resolution, or a Mac-specific one, as you won't find it on your PC. Since Macs and PCs these days quite often use the same video hardware, chances are that both are true.
2. You will also find 96 dpi on PC hardware running non-Microsoft operating systems. For the reasons stated above, I still don't understand why 96 dpi is used. 72 dpi is much more useful for achieving WYSIWYG printed output for anything with text on it.
