Circle of Fifths
The Circle of Fifths is a visual representation of the relationship between the various major and minor keys in music.
It shows how each major key in western music can be constructed from any other such key by transposing upwards by seven semitones (or downwards by five semitones) a number of times.
Seven semitones is the interval called a "perfect fifth" in music, and after twelve such transpositions we return to the original key. Thus the sequence forms a "circle of fifths".
If we begin with the major key of C, with no sharps or flats, each transposition upwards adds an extra sharp to the key signature. Alternatively, moving downwards from the key of C, each transposition adds one flat.
A diagram of the Circle of Fifths is available on Wikipedia Commons at http://upload.wikimedia.org/wikipedia/commons/3/33/Circle_of_fifths_deluxe_4.svg