What is a hertz?
the derived unit of frequency
What does it measure?
sine waves
Anything else?
yes
Like what?
also  the speeds at which computers are driven.
How did it get its name?
It is named for Heinrich Rudolf Hertz
Who is he?
the first person to provide conclusive proof of the existence of electromagnetic waves
Is it used only in English-speaking countries?
no
How do you know that?
it is in the International System of Units (SI)
What time span is used in defining the measurement?
a second
How long is a second?
"the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom"
Who decided that?
The International Committee for Weights and Measures