Hi Teemu,
REST API expose screens/programs/tables "as is" via GET for getting info, or POST for performing operation on the model.
REST API are also statefull vs the host session.
Restful services allows to defined a "contract", meaning a well defined input/output, which can get/update information from multiple screens/programs/tables.
These service are statefull vs. the host session, and usually uses session pooling.