It's "fair usage". They reserve the right to remove or terminate unlimited data if you use too much, if you don't believe me you can go out of your way to use more than 1 GB. It may or may not be exactly 1GB (not sure because I haven't asked them yet) but they do have fair usage claims right here in the ToS
Everything will work the same. This is a reseller of T-Mobile service, they have no unlimited data buckets. Datajack, a company that was reselling resold T-Mobile data got shut down quickly with it's "unlimited" data model and they don't even sell T-Mobile data anymore, they use Sprint.
http://www.mysimplemobile.com/Simple-Mobile-Terms-Conditions.aspx