UNDERSTANDING KEY SIGNATURES:
When musicians talk of scales, or even a piece of music as being in a certain key; i.e., this song is in the key of, F Major... they are defining the key signature and tonality off of the tonic note, (the, tonic, is the key note of a scale), and the specific notes (and to a lesser extent the chords), found within the piece. The altered tones found in the scale are the signature, the tonality is the key's center, (or Key Center).

For example:
If we say a certain melody is in the key of
G Major, then the melody is made up
of notes from the, G Major, scale;
G, A, B, C, D, E, F#.

The tonic note, (or first note - key note), of the scale is; G, but the key center is G Major.

Key signatures are given at the beginning of a piece of music. They are shown as the appropriate sharps or flats on the staff for the prescribed key. The sharps and flats are indicated between the clef and time signature. When placed on the staff in this manner we call this the, Key Signature.

For more details on this post (including a FREE handout) follow the link below and visit the Creative Guitar Studio website for this lesson plan:
http://www.creativeguitarstudio.com/lessons/music_theory/key_signatures.php