The Pay system is a payment solution for online merchants who wish to accept payments using local bank transfers, which is pretty common in Germany and other European countries. Like PayPal, without the intermediary.
This is basically a clone of the functionality offered by Sofort. Except for now it only supports Deutsche Bank.
The functionality exposed by the frontend is all the final user views, while the webservice is necessary for online shops implementing this solution. They work together and access the same database, although they run in separate servers.