Portable Pix Map, an image format of the PNM family. It is a part of PBM Plus - a large numbers of image converters and manipulators available on the internet.
Every pixel is represented by three bytes for red, green, and blue channel. No compression, no transparency, as far as RS is aware.
Tk images can be read and written in PPM format using photo [as of which version?]. See also PGM.