In MS-DOS, .com was the extension of executables - as far as I know, .com files were just raw binary with no headers, so they could be created with things like debug easily. IIRC, they were also limited to 64 kilobytes in size, which is why they were later largely replaced with .exe files. DOS 6 had only a handful of COM files.

One of the most widely remembered .com file is, of course, COMMAND.COM. (Also registered as a domain name? You betcha. =)