Google Font CDN
google.com
google.com
To use this feature choose a font and click on the little 'pop out' icon (that looks like one square on top of another) and then click "Pairings". Here's an example: https://www.google.com/fonts/specimen/Lobster#pairings
According to Wikipedia [1], a CDN is simply "a large distributed system of servers deployed in multiple data centers across the Internet".
When you use a car, you're usually driving on public roads. But you don't say "hey honey, would you like to use the road to get some chinese food?" because the application you (the user) are using directly is not the road, it's the car. The car just happens to use the road as a means of interconnection from home to the chinese takeout place.
There can be many kinds of distributed systems of servers deployed in multiple data centers across the Internet to serve content to end-users. But often these serve other functions besides just distributing content. The underlying principle of a CDN is that it just moves content around, regardless of what it is.
With Google Fonts, you aren't using a tool to move content around. You're using an API to control an application that manipulates fonts. It just happens to be running on a network that distributes the content around in the background. Google Fonts uses a CDN to give you content, but it's far more complex than a typical CDN, and your use of it is merely to distribute content, but to specifically access and manipulate fonts.