Wikipedia has a comprehensive article on Unicode or if you want to know all the details go to the Unicode Consortium. Most, maybe all, standard Apple fonts are Unicode and AFIK all the current Apple apps and a substantial number of third party apps can handle Unicode as well. Older apps such as Appleworks are not Unicode aware and therefore have only a very limited character set available.

If you want to see the Unicode characters in Character Viewer on the View drop down at the top of the Character Viewer window select "All Characters".


"All you've got to do is own up to your ignorance
honestly, and you'll find people who are eager to
fill your head with information"
--Walt Disney