Source : Free On-Line Dictionary of Computing

Portable Pixmap
     
         (PPM) A colour {image} {file format}.
     
        A PPM file contains the following:
     
         a two character "{magic number}" - "P3",
         the width in pixels,
         the height in pixels,
         the maximum colour component value,
         HEIGHT rows of WIDTH {pixels}.
     
        The rows are ordered from top to bottom with the pixels in
        each row ordered from left to right.  Each pixel is
        represented as three values for red, green, and blue.
     
        All parts are separated by {whitespace} and numbers are in
        decimal {ASCIII} representation.  A zero pixel component means
        that colour is absent.  Characters from a "#" to the next
        end-of-line are ignored and no line should be longer than 70
        characters.
     
        Here is an example of a small pixmap in this format:
     
         P3
         # feep.ppm
         4 4
         15
          0  0  0    0  0  0    0  0  0   15  0 15
          0  0  0    0 15  7    0  0  0    0  0  0
          0  0  0    0  0  0    0 15  7    0  0  0
         15  0 15    0  0  0    0  0  0    0  0  0
     
        A "RAWBITS" variant has magic number "P6", pixel values are
        stored as plain binary bytes, instead of ASCII decimal and no
        whitespace is allowed after a single whitespace character
        after the maximum colour component value which must be less
        than or equal to 255.
     
        (1998-02-06)