PK files (packed raster) are the canonical form of TeX font bitmaps. The output from MetaFont includes a generic font (GF) file and the utility gftopk produces a PK file from that.
There are potentially a lot of PK files, as one is needed for each font: that is for each magnification of each design (point) size for each weight for each font in each family.
Further, since the PK files for one printer do not necessarily work well for another, the whole set needs to be duplicated for each printer type at a site.
While this menagerie of bitmaps can (in principle) provide fonts that are closely matched to the capabilities of each printer, the size of the collection (and the resulting difficulty of maintaining it) has been a potent driver to the move towards outline fonts such as Adobe Type 1 fonts.
This answer last edited: 2012-10-20