As a computer programmer I have no problem understanding labels/tags.
However I find it very hard to make people understand that an email can have more than one label.
Also I find it hard to explain that you can still get to an email if it is not in your inbox and does not have a label.
(Moving a email to a folder seem to be how most people think)
Is gmail just designed by computer programmers for computer programmers? (Hence why I like it so match)
Just tell them that every e-mail is like a movie. The same movie can belong to more than one categories/genres like