The presentation of typeface families on the desktop is based on family and style names in the font files. Operating systems interpret these names into menu selections to make them easier for people to use. On the web, typeface families and styles have been identified generically, or by file name, and then assigned properties and called whenever a run of text requires that family.
If a “font family” is just one one style, the file contains the whole family. But even if a font family contains just weights and widths, names and menus can start to bewilder minds and stretch user interfaces: OldDreadful-ExtraBoldCondendsedItalic. And more complex families—those including extremely large style menus, size masters, “pro” glyph sets, weights, widths, and italics in the same family—have forced font developers to break the family into subfamiily names like OldDreadfulText-ExtraBoldCondendsedItalic for easier use via subfamily grouping in menus.