(For instance, if you buy 3 GB of data for a month, that would cost an additional $30 -- $10 times the 3 GB. But if you only use 1.4 GB of that, you'll get $16 back.)
Google won't be building its own wireless network, but has a deal for access to piggyback on the networks of Spring and T-Mobile.
There's no start date yet from the program, which right now they call Project Fi. At the moment, it is by invitation only, which you can sign up for at Google here, and at first it will only be available for Google's own Nexus 6 phone.
You can read more information about the program in Google's announcement.