What is a USB 'header' (and how is it different from a port)?

With the prevalence of USB devices, it might be easy to mix up USB headers and ports, but there is a key difference between ...
SlashGear

What Are USB Headers And How Are They Different From USB Ports?

If you're upgrading or building your own PC, you'll probably find some components inside that are similar in size and shape to USB ports. But those are actually USB headers, and even though they look ...
Ars Technica

Does anyone know of a USB add-in card that has internal connectors?

Hi all!<P>I'm going to be rebuilding a computer soon. And the motherboard I'll be replacing has a pair of internal USB headers, to which I've connected an Antec Easy USB bay adapter.<P>But the new ...