The Netpbm formats are a family of simple raster image file formats, associated with the Netpbm library and suite of image processing utilities.
PBM (portable bitmap) is a bi-level image format.
PGM (portable graymap) is a grayscale image format.
PPM (portable pixmap) is a color image format.
PNM (portable anymap) is a collective name for PBM, PGM, and PPM. A .pnm file may use any of those formats.
PAM (portable arbitrary map) is a more sophisticated format which supports everything the PBM, PGM, and PPM formats do, and then some. Some resources incorrectly name this format "Portable Any Map".
Each file begins with two ASCII characters, which serve as a signature.
The PBM, PGM, and PPM formats come in two varieties: a text ("plain") format, and a binary ("raw") format. PAM is binary-only.