This is officially termed the ICAO (International Civil Aviation Organization) code. The ICAO is a United Nations department. Initially used by the US military, the code has since gained pervasive adoption by the airlines and is used throughout all civil aviation.
The code is meant to provide unique sounding words for each letter so that reading alphabetic letters over radio waves will not be misunderstood. I find it useful for telephone conversations and other earth-bound situations. 
    A=Alpha
    B=Bravo
    C=Charlie
    D=Delta
    E=Echo
    F=Foxtrot
    G=Golf
    H=Hotel
    I=India
    J=Juliet
    K=Kilo
    L=Lima
    M=Mike
    N=November
    O=Oscar
    P=Papa
    Q=Quebec
    R=Romeo
    S=Sierra
    T=Tango
    U=Uniform
    V=Victor
    W=Whiskey
    X=X-ray
    Y=Yankee
    Z=Zulu