What is PNG?
PNG is the web's lossless image format: compression that never degrades the picture, and a real alpha channel for transparency. It is the right choice for screenshots, logos, line art and anything with sharp edges or text.
What is PBM?
PBM is Netpbm's bilevel format - one bit per pixel, pure black or pure white, nothing in between. It is used for scanned documents, fax-era imagery and dithered art, and its packing convention surprises people: a 1 bit means black.