Major-minor tonality in the common practice is organized into twelve major keys and twelve minor keys. Each key collects up a set of chords and progressions which emphasizes movement away from and back to the tonic, a chord sharing the name of the key. More
There are fifteen major and fifteen minor named keys for historical tuning reasons (before the advent of twelve-tone equal temperament) and as a quirk of musical notation. In twelve-tone equal temperament, three keys have two names and two sets of notation. These three keys are considered six different keys in historical tuning systems, where instruments were tuned in such a way as to limit each piece of music to a small number of keys.
See Circle of Fifths & Keys for more on the ordering of the keys and mnemonics for their note names.