General XPCOM question

Hi all,

I have some COM background, and I'm interested to learn more about XPCOM.

So far, I understand that XPCOM stands for Cross Platform COM.
But, I didn't understand what this really means.

How exactly the "Cross Platform" goal is achieved?
Or, in other words, what rules should I obey (as a XPCOM componet 
programmer) in order to build a "Cross Platform" component?

Thanx,
Adrian.