A filename extension is a suffix to the name of a computer file applied to indicate the encoding convention (file format) of its contents. In some operating systems (for example Unix) it is optional, while in some others (such as Windows) it is a requirement. Some operating systems limit the length of the extension (such as DOS and OS/2, to three characters) while others (such as Unix) do not. Some operating systems (for example RISC OS) do not use file extensions.
Here you have most popular extensions:
.$$$ Temporary file
.$$A OS/2 program file
.$$F OS/2 database file
.$$S OS/2 spreadsheet file
.$D$ OS/2 planner file
.$DB DBASE IV temporary file
.$ED Microsoft C temporary editor file.
.$VM Microsoft Windows temporary file for virtual managers.