The primary responsibility of the Database Writer process(es) is to write (dirty) buffers from the buffer cache to disk.
The n in DBWn indicates that there might be more than one database driter processes. The number of processes can be configured with the db_writer_processes init parameter.
The names of the first 36 is DBW0, DBW1, … DBW9, DBWA, DBWZ. The 37th to the 100th are named BW36, DB37, … BW99.