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

Using USB 2.0 Motherboard Header with Case that Doesn't Support USB 2.0 (Front USB Ports)?

USB 1.1 and 2.0 appear to share the same pin header layout on my motherboard (Abit AT7 with both 1.1 and 2.0 headers). I'm assuming that the wires are able to carry the same signals as 2.0, so I think ...
Ars Technica

What's the technical name of a motherboard USB header connector?

The USB A female connectors were easy to find on Digi-Key's website. However I'm not sure what the technical name is for the motherboard header for USB. Anybody know? Or alternatively, anyone have a ...