Source : Free On-Line Dictionary of Computing

base 64
     
         A file format using 64 {ASCII}
        characters to encode the six bit {binary data} values 0-63.
     
        To convert data to base 64, the first byte is placed in the
        most significant eight bits of a 24-bit buffer, the next in
        the middle eight, and the third in the least significant eight
        bits.  If there a fewer than three bytes to encode, the
        corresponding buffer bits will be zero.  The buffer is then
        used, six bits at a time, most significant first, as indices
        into the string
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
        and the indicated character output.  If there were only one or
        two input bytes, the output is padded with two or one "="
        characters respectively.  This prevents extra bits being added
        to the reconstructed data.  The process then repeats on the
        remaining input data.
     
        Base 64 is used when transmitting binary data through
        text-only media such as {electronic mail}, though {uuencode}
        is more common.
     
        (1995-03-21)