A TeX font metric, or TFM, is a font metric format used by TeX. Unlike outline font formats such as TrueType, a TFM provides only the information necessary to typeset the font: each character's width, height and depth. The actual glyphs are stored elsewhere. This is not unique to TeX; Adobe's AFM files and Windows' PFM files use the same technique.
TeX uses only TFM files to produce its output DVIs. The actual glyphs are inserted (from, for instance, METAFONT's PK packed fonts) by the eventual DVI output driver or previewer.
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