Many different kinds of files are available on the net. These files contain many different kinds of data for many different architectures. Frequently, the extension (trailing end) of a filename gives a good clue as to the format of its contents and the architecture that it was created on.
In order to save space, most files on the net are compressed in one way or another. Many compression/decompression programs exist on multiple architectures.
Multiple files and directories are often combined into a single `archive' file. Many archive formats perform compression automatically.
Unix `tape archive' format. Tar files can contain multiple files and directories. Unlike most archiving programs, tar files are held together in a wrapper but are not automatically compressed by tar.
Unix `compress' format. Compression doesn't form a wrapper around multiple files, it simply compresses a single file. As a result, you will frequently see files with the extension .tar.Z. This implies that the files are compressed tar archives.
GNU zip format. GNU zip doesn't form a wrapper around multiple files, it simply compresses a single file. As a result, you will frequently see files with the extension .tar.z or .tar.gz. This implies that the files are compressed tar archives. Do not confuse GNU Zip and PKZip or GNU Zip and Unix compress, those are three different programs!
Macintosh `BinHex' format. In order to reliably transfer Mac files from one architecture to another, they are BinHex encoded. This is actually an ascii file containing mostly hexadecimal digits. It is neither a compression program nor an archive format.
Macintosh `Stuffit' archive.
Macintosh `Compactor' archive.
Like the .tar.Z format that is common among Unix archives, Macintosh archives frequently have the extensions .sit.hqx or .cpt.hqx indicating a BinHex'ed archive.
PC `arc' archive. This is an older standard (in PC terms, at least) and has gone out of fashion.
PC `zip' archive. This is the most common PC archive format today.
PC `arj' archive.
PC `zoo' archive
PC `lha/lharc' archive.
`UUencoding' format. In order to reliably transfer binary data across architectures (or through email), they are frequently uuencoded. This is actually an ascii file. It is neither a compression program nor an archive format.
Just as the are many, many archive formats, there are many different font formats. The characteristics of some of these formats are discussed below. Once again, the file extension may help you to determine the font type. (On the Mac, the resource TYPE field is (probably) a better indicator).
Postscript Type 1 fonts (Also called ATM (Adobe Type Manager) fonts, Type 1, and outline fonts) contains information, in outline form, that allows a postscript printer, or ATM to generate fonts of any size. Most also contain hinting information which allows fonts to be rendered more readable at lower resolutions and small type sizes.
Postscript type 3 fonts are an old outline font format that is not compatible with ATM. Most developers have stopped using this format except in a few special cases, where special type 3 characteristics (pattern fills inside outlines, for example) have been used.
Truetype fonts are a new font format developed by Microsoft with Apple. The rendering engine for this font is built into system 7 and an init, the Truetype init, is available for system 6 (freeware from Apple). It is also built into MS Windows v3.1. Like PostScript Type 1 and Type 3 fonts, it is also an outline font format that allows both the screen, and printers, to scale fonts to display them in any size.
Bitmap fonts contain bitmaps of fonts in them. This a picture of the font at a specific size that has been optimized to look good at that size. It cannot be scaled bigger without making it look horrendously ugly. On the Macintosh, bitmap fonts also contain the kerning information for a font and must be installed with both type 1 and type 3 fonts. Their presence also speeds the display of commonly used font sizes.
Adobe Type 1 metric information in `ascii' format (human parsable)
Bitstream compressed outline
Adobe's Bitmap Distribution Format. This format can be converted to the platform specific binary files required by the local X Windows server. This is a bitmap font format distributed in ASCII.
Bezier outline information
Borland stroked font file
Sun formats. More info when I know more...
MS-Windows TrueType format fonts
Generic font (the output of TeX's MetaFont program (possibly others?))
Font libraries produced by emTeX fontlib program. Used by emTeX drivers and newer versions of dvips.
TeX MetaFont font file (text file of MetaFont commands)
Adobe Type 1 Postscript font in ASCII format (PC/Unix) I believe that this format is suitable for directly downloading to your PostScript printer (someone correct me if I'm wrong ;-)
Adobe Type 1 PostScript font in ``binary`' format (PC/Unix) Note: this format is not suitable for downloading directly to your PostScript printer. There are utilities for conversion between PFB and PFA (see the utilities section of the FAQ).
Printer font metric information in Windows format
TeX packed bitmap font file (also seen as .###pk where ### is a number)
TeX `property list' file (a human readable version of .tfm)
Frequently, any PostScript file. With respect to fonts, probably a Type3 font. This designation is much less `standard' than the others. Other non-standard extensions are .pso, .fon, and .psf (they are a mixture of type 1 and type 3 fonts).
TeX pixel bitmap font file (obsolete, replaced by .pk)
LaserJet bitmapped softfont, landscape orientation
LaserJet bitmapped softfont, portrait orientation
LaserJet scalable softfont
TeX font metric file. Also an HP Tagged Font Metric file.
TeX virtual font which allows building of composite fonts (a character can be composed of any sequence of movements, characters (possibly from multiple fonts) rules and TeX specials)
TeX `property list' (human readable) format of a .vf