The QC is the session that initiates the parallel SQL statement and the PX servers are the individual sessions that perform work in parallel.
The QC distributes the work to the PX servers and may have to perform a minimal – mostly logistical – portion of the work that cannot be executed in parallel.
For example a parallel query with a SUM() operation requires a final adding up of all individual sub-totals calculated by each PX server.
The QC is easily identified in a parallel execution plans as 'PX COORDINATOR'. The process acting as the QC of a parallel SQL operation is the actual user session process itself.