we refer to a specific font family (all the letters, numbers, signs etc in that font)
>lax usage

Using HTML styles you can assign a "font-family" to particular items or pages or whatever. But in this context you would have something like
font-family: Verdana, Arial, Sans-Serif
If the browser machine didn't have the Verdana font available it would use Arial; if it didn't have Arial it would use whatever sans serif font it happened to have around.

So a font family is whatever the Web developer considers a group of equivalent fonts.

An even more lax usage, shanks!