In short, all characters are encoded as numbers for computers to process. Unfortunately, not all programs/websites use the same character encoding, so one number may represent different characters from program to program or site to site. That can cause issues like the one you encountered when one program/website does not know which character the other program/website is referring to.
CNET is actually having problems with character encoding right now, so you'll notice that quotation marks, amongst other characters, may not appear correctly if you copy-and-paste them into the forums. The engineers are working on the problem to correct what you're seeing here, but there's little you can do regarding other websites beyond ensuring your browser's character encoding is set to UTF-8 or your localized default.
In IE7, when reading a page I see (as an example):
???Yeah, that???s what I believe
In Firefox, this is what it looks like:
?Yeah, that?s what I believe
My company requires IE7 for my work, so I cannot just use Firefox.