What is PBM?

One bit per pixel - black or white, nothing in between. The most minimal image format in common use, and still the right one for scanned documents, line art and anything headed into OCR or shape analysis. Photographs do not survive it.

What is BMP?

The Windows bitmap, which stores pixels raw with no compression worth mentioning. Files are enormous. It survives because it's trivial to read - if you're writing an image loader from scratch or feeding an embedded display, BMP is the format that needs no library.