difference between 'where' and 'having'

Hi, I have a question, that's probably really stupid, but could someone please explain to me what difference there is between a WHERE clause and a HAVING clause besides the syntax? I read the documentation where it states that:

Expressions in the HAVING clause can refer both to grouped expressions and to ungrouped expressions (which necessarily involve an aggregate function).

I've been happily working with Postgres for a long time, and not once have I ever used a HAVING in my queries, even in fairly complex ones, and I just can't bear the suspense anymore! :)

Thanks in advance for clearing this up for me.

Adam