Each font is free for commercial use – and you can download them for desktop usage too (Google did all the homework for you).
Of course, the choices available aren't premium like the typefaces you can purchase for hundreds of dollars – but if you're on a budget, choosing from it's huge library (703 families and counting!) is a great thing to do.
Its use is simple:
When you click on the font you like, the window will enlarge showing you new buttons on the lower-right.
Clicking the Pop out one will allow you to focus more on the font at first, to see some statistics about its usage (country, number of downloads, and so on) and more importantly, in the Pairings tab you'll be able to see some pre-made pairings of...