Thee are web-safe fonts that are standard across all operating systems and platforms.These are therefore built into the browsers on the platform with recourse to any other on-line asset.
If these are not used, the collective term might be "non-standard".
We are talking about Arial, Helvetica, Times & TNR, Courier & C New, Verdana, Georgia, Palatino, Garamond, ...
If not available the browser picks the next best thing.
Not so goo for your finely-tuned website.

Web Fonts to the rescue.
These are "installed" into your web page on the fly with a small bit of code in the header.
Basically, this is a CSS font-family expression and the code is just a link to the web font.

Acorn