WPS is a stateless technology - you give it a process to execute (by HTTP GET or POST), and after some time, it gives you some sort of result. This may be a bit of XML or JSON, a link to an image, or the image itself.
On the server side, someone will have written one or more processes, detailing the expected inputs and the given outputs. The language these are written in are transparent to you the user. In the case of pyWPS they'll be written in Python. With the ZOO-project, it can be in one of a multitude of languages. But each process is completely standalone from any other process. Also, there is no standard set of processes that have to be created - so you are entirely at the whim of the WPS server you are connected to.