Key Signature
The short version
The sharps or flats written at the start of the music.
It's the shorthand on the page that says "every F in this piece is F♯", so nobody has to write the symbol a hundred times.
0 sharps/flatsC major or A minor
1 sharp (F♯)G major or E minor
1 flat (B♭)F major or D minor
Same signature, two possible keys
Why you'd careIf you read or write notation, the key signature saves you from a mess of accidentals — and tells you the likely key at a glance.
Go deeper
Key and key signature are related but not the same. The key is what your ear hears as home. The key signature is notation: it's a set of sharps or flats, and one signature can serve two keys — no sharps or flats means C major or A minor. The signature can't tell you which; only the music can.
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