1. Google fonts are all those fonts which are available at https://fonts.google.com/ and can be used in typography just by writing the name of that font in google font when you click on it.2. Custom fonts are those fonts which are availabale in WOFF, TTF etc files.3. System fonts are those fonts which are installed and available in the system.When they all are used in typography, first preference is given to Google Web fonts if internet access is available, if not then custom fonts are used. If custom fonts are unavailable then system font is applied while running a website.