What does a Vector Network Analyzer actually measure???

[Joe]

Hello there, I have a quick question.

I'm an undergraduate physics major learning a few basics on Microwave
theory with regards to communications. I've been reading many papers
and I understand what S-parameters are and signal analysis along
transmission lines. My question is quite fundamental.

What does a Vector Network Analyzer (VNA) actually measure? All the
papers and tutorials I've been reading describe the VNA as measuring
the incident & reflected/transmitted signal, but none actually have
said what that signal actually is. Is it a voltage, current, or power
signal?? Or is it something else??

Thanks for your time

Joe