Class and ID

I've been reading the book on Cascading Style Sheets by Lie and Bos.
Could someone please explain the difference between .class and #id,
and give an example of when one would like to use one instead of the
other? Perhaps I'm a bit thick but I don't really get the difference.

Martin S.
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Martin Skjöldebrand
System administrator and archaeologist!
http://www.bahnhof.se/~chimbis/tocb/