Chapter 7. Fonts and Encodings
Half of the job of (La)TeX as a typesetting system is to process the source document and to calculate from it the characters’ positions on the output page. But (La)TeX has only a primitive knowledge about these characters, which it basically regards as black boxes having a width, height, and depth. For each font these dimensions are stored in a separate external file, the so-called TeX font metric or .tfm file.
The character shapes that correspond to such a .tfm file come into play at a later stage, after (La)TeX has produced its .dvi file. Character placement information in the .dvi file and information about character shapes present in the .pk file or in outline descriptions (e.g., PostScript) ...