If you keep maxdop at 0, then there can be as many active threads for each query as you have cores. If you put maxdop to 4, then each query can only have 4 executing threads at a time.
MAXDOP does not limit number of threads per query, only the number of concurrently executing threads.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability