Source : Free On-Line Dictionary of Computing

uuencode
     
        Unix program for encoding binary data as ASCII.  Uuencode was
        originally used with uucp to transfer binary files over serial
        lines which did not preserve the top bit of characters but is
        now used for sending binary files by e-mail and posting to
        {Usenet} newsgroups etc.  The program uudecode reverses the
        effect of uuencode, recreating the original binary file
        exactly.
     
        Uuencoded data starts with a line of the form
     
        	begin  
     
        where  is the files read/write/execute permissions as
        three {octal} digits and  is the name to be used when
        recreating the binary data.
     
        Uuencode repeatedly takes in a group of three bytes, adding
        trailing zeros if there are less than three bytes left.
        These 24 bits are split into four groups of six which are
        treated as numbers between 0 and 63.  Decimal 32 is added to
        each number and they are ouput as ASCII characters which will
        lie in the range 32 (space) to 32+63 = 95 (underscore).  Each
        group of sixty ouptut characters (corresponding to 45 input
        bytes) is output as a separate line preceded by an 'M' (ASCII
        code 77 = 32+45).  At the end of the input, if there are N
        output characters left after the last group of sixty and N>0
        then they will be preceded by the character whose code is
        32+N.  Finally, a line containing just a single space is
        output, followed by one containing just "end".
     
        Sometimes each data line has an extra dummy character added to
        avoid problems which mailers that strip trailing spaces.
        These characters are ignored by uudecode.
     
        Despite using this limited range of characters, there are
        still some problems encountered when uuencoded data passes
        through certain old computers.  The worst offenders are
        computers using non-ASCII character sets such as EBCDIC.