There is one work generator per application. It creates workunits and the corresponding input files. It is application-specific, and uses BOINC-supplied interfaces for registering the workunits in the database.
During testing, you can create a single workunit using create_work, then use make_work to make copies as needed to maintain a supply of work.
This program (which is supplied by BOINC and is application independent) handles various state transitions of workunits and results, such as timeouts. It generates initial results for workunits, and generates more results when timeouts or errors occur.
There is one validator per application. It compares redundant results and selects a canonical result representing the correct output, and a canonical credit granted to users and hosts that return the correct output.