(For another perspective on this, see section `Font concepts' in Dvips).
Font files come in several varieties, with suffixes like:
.tfm .*pk .*gf .*pxl (obsolete) .pl .mf .vf .vpl
Each represents a file format.
A TFM (TeX font metric) file is a compact binary file that contains information about each character in a font, about combinations of characters within that font, and about the font as a whole. The font metric information contained in TFM files is device-independent units is used by TeX to do typesetting. Unlike the bitmap (raster) fonts described below, TFM font files contain no information about the shapes of characters. They describe rectangular areas and combinations thereof, but not what will eventually be printed in those areas.
Since TeX does scaling calculations, one TFM file serves for all magnifications of a given typeface. On the other hand, the best printed results are obtained when magnified (or reduced fonts) are not produced geometrically (as done by PostScript, for example) but rather optically, with each size a separate design (as done with Computer Modern and the DC fonts, for example); then a separate TFM file is needed for each size.
At any rate, TeX produces a DVI (DeVice Independent) file from your source document. In order to print DVI files on real devices, you need font files defining digitized character shapes and other data. Then previewers and printer-driver programs can translate your DVI files into something usable by your monitor or printer. Bitmap fonts come with suffixes such as `.600pk' or `.600gf' or `.3000pxl', where the `600' is the horizontal dots-per-inch resolution at which the font was produced, and the `pk' or `gf' or `pxl' indicates the font format. Outline fonts in PostScript Type 1 format have suffixes such as `.pfa' or `.pfb'.
Fonts in pk (packed) format are in the tightly packed raster format that is pretty much the standard today. They take up less space than fonts in the gf (generic font) format that Metafont generates, and far less space than fonts in pxl format. Fonts in pxl format take up gross amounts of disk space and permit only 128 characters. They are obsolete.
Font files with the `.pl' (property list) suffix are the plain text (human-readable) analog of the binary `.tfm' files. The TFtoPL and PLtoTF programs convert between the two formats (see section TFtoPL: TeX font metric to property list conversion and section PLtoTF: Property list to TeX font metric conversion).
Font files with the `.mf' suffix are in Metafont source format. These are the files used by Metafont to generate rastered fonts for specific typefaces at specific magnifications for the specific resolution and type of mapping used by your device.
The suffix `.vf' identifies "virtual font" files, for which `.vpl' is the human-readable analog. See section VFtoVP: Virtual font to virtual property lists and section VPtoVF: Virtual property lists to virtual font. For further discussion of virtual fonts, see `CTAN:/doc/virtual-fonts.knuth', `CTAN:/help/virtualfonts.txt', and section `Virtual fonts' in Dvips.
(This section is based on documentation in the original Unix TeX distribution by Pierre MacKay and Elizabeth Tachikawa.)
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