Difference between function and procedure?

Hi,

I'm diving more into depth of the features outside the typical table
stuff, which I usually need.

I wrote a trigger that fires a function written in PL/Python, both works
fine so far.

But as I browsed my database via pgAdmin3 I recognized that there is
also 'Procedures' under 'public' available, but empty yet. Trying to
create a procedure resulted in the same SQL statement like creating a
function (CREATE FUNCTION ...). I couldn't see any difference. Also
didn't find in the PostgreSQL manual if there is a difference between
functions and procedures. Are there any at all?

Anastasios

PS: Using PostgreSQL 8.1.3 on Windows XP development machine.