PIAA H3 and H4 Bulbs... Are they universal?

I've always bought some Xenon Ebay bulbs for my 3G Eclipse and have noticed this H3 and H4 stuff on them. But I heard from this guy at a Car Parts Shop that PIAA's are the best for bulbs. I checked out their site http://www.piaa.com and they got some wicked sick bulbs that are about as bright as stadium lamp posts.

From what I think or assume I understand, H3 is probably lightbulbs for fog lights and H4 is for headlights...

But my question is are they universal for any other car? So if I would take an H4 from a 2000+ Eclipse it would fit in say... a Ford Focus?

Or put simply are H4's Compatible with other cars that have H4 for headlights? Does the same go for Foglights ( H3 )?

If my car accepts H4's can it accept PIAA's H4's and the same for H3?

If H4 = Circle
and Circle = Circle
Then H4 = H4

Is the above correct?

Thanks

Yes.