The definition of extension

Filename extensions originate back in the days of CP/M and basically allow a very rough grouping of different file types by putting a tag at the end of the name. To further complicate matters, the tag is sometimes separated by the name proper by a period "." and sometimes by a tab. While extensions are semi-enforced on CP/M, MS-DOS, and MS-Windows, they have no real meaning aside from convention on other platforms and are only optional.