A key is to do with the overall song. I will make examples of a song that stays in the same key, with no "accidentals" for now. The key determines what notes are used in a song. If I say a song is in the key of C, then this means that the melody is written using notes of the C major scale, and all chords must be made from notes of the C major scale. The key signature is what you see at the start of a piece of written music, where there are # or b pictures (or nothing in the key of C) just before the time signature (ie, 4/4) and just after the clef (usually treble for guitar). The key signature is aa way of showing what key the song is in by showing how many #s or bs there are. The sharps and flats are put on the lines where those notes are sharpened or flattened, so in the the key of G, you would see one # on the top line of the stave - to designate an F#. Also, this means that wherever you see the note F written, you should play an F#, unless otherwise noted. This makes the page look neater, because if the song stays in key there will be no # or b symbols on the page.