Digital electronic design resources?

[Peter Korsten]

This may be a bit of a long shot, but I was wondering if there's 
something that teaches you how to design simple boards like the Arduino, 
without having to do a course.

I understand digital signals (it's the only course at university where I 
actually had a decent mark), but what I don't understand is why there 
should be a resistor here or a capacitor there.

So... how difficult is it to learn such a thing?

- Peter