system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: What does it mean when music is played in a certain "key"

I understand musical notes but nobody has been able to explain to me what a "key" is in music. 

For example, how does "beethovens 3rd symphony in e-minor" differ from "beethovens 3rd symphony in f-minor"?
gpt
A key has two parts - the selection of notes you use, and the specific note you choose to be the "tonal center" or home note of your piece.  There are 12 notes in all, and you typically choose 7 of them to make up your scale.  If you think of the numbers on a clock, and you mark 7 of them as "your" notes according to a pattern\*, and one as your tonal center, you can get an idea of how that works.  The tonal center is the one you typically come back to in order to give a sense of finality to your musical phrase or passage.  Anyway, in the examples you use, E and F are the tonal centers, and the "major" or "minor" indicate the pattern by which you choose your 7 notes.  In E minor, you'd center on E, and choose your 7 notes based on a "minor" pattern\*.  In F minor, you'd center on F and choose your 7 notes based on the same "minor" pattern.