There are two plans. An unlimited domestic plan for those that don’t travel outside of the US, and a roaming plan for those that travel. The unlimited plan offers unlimited calls, SMS, and data for $40/month. Those that hit 20GB of data will be throttled when the network is congested.
The roaming plan is $15 for unlimited voice/SMS and $5/GB of data. When traveling to partner carriers in other countries, the service will give you 3G speeds for data at the same rate as domestic.