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 ...
Ars Technica

Are all micro-usb cables interchangeable?

I've got some old cords that have a micro-usb port on one end and a wall plug on the other. These are for charging mobile phones I used to have. Should I toss them or save them for future phones I'll ...
Ars Technica

Can you attach multiple USB-C plugs to a single motherboard header?

I've got a motherboard that has a single USB-C header on it. I also have a case with no USB-C ports on the front panel. I know things like this exist, but I'm wondering is, is there anything with ...