There Ain't No Such Thing As Plain Text

Joel concludes: If you completely forget everything I just explained, please remember one extremely important fact. It does not make sense to have a string without knowing what encoding it uses. You can no longer stick your head in the sand and pretend that “plain” text is ASCII.

If you thought plain text is just ASCII or only vaguely know about character encodings, you might want to read this article on Unicode from Joel Spolsky and freshen up on the subject.