Actually,
ASCII is a
7-bit standard.
Characters 128 through 255 are
vendor-specific. The
IBM PC and
clones use a
buttload of these characters for
line drawing, whereas the
Macintosh uses most of them for
accented
letters.
Some of the characters the Mac has and the vanilla DOS PC doesn't: Portuguese vowels; the ae, oe, fl, and fi ligatures; smart quotes and European quotes; the dagger and double dagger marks; and the em dash.
If you haven't had enough weird characters in vendor-extended ASCII, check out Unicode.