The MAXDOP query hint is only applicable on systems and SQL Server editions for which parallel plans are possible. On single-core systems, multiprocessor systems where CPU affinity has been set to a single processor core, or systems that don’t support parallel plans (i.e. if you are running the express edition of SQL Server which can only utilize a single processor core), this query hint has no effect.
On systems where parallel plans are possible, and in the case of a query where a parallel plan is being generated, using MAXDOP (n) allows the Query Optimizer to use only n workers.
On very large SMPs or NUMA systems, where the SQL Server configuration setting for Max Degree of Parallelism is set to a number less than the total available CPUs, this option can be useful if you want to override the systemwide Max Degree of Parallelism setting for a specific query.
A good example of this might be a 16 core SMP server with an application database that needs to service a large number of concurrent users, all running potentially parallel plans. To minimize the impact of any one query, the SQL Server configuration setting Max Degree of Parallelism is set to 4, but some activities have a higher “priority” and you want to allow them to use all CPUs. An example of this might be an operational activity such as an index rebuild, when you don’t want to use an online operation and you want the index to be created as quickly as possible. In this case, the specific queries for index creation/rebuilding can use the MAXDOP 16 query hint, which allows SQL Server to create a plan that uses all 16 cores.
Before using any query hints, run a web search for the latest information on issues with query hints. Try searching on the keywords “SQL Server Query Hints” and look specifically for anything by Craig Freedman, who has written several great blog entries on some of the issues you can encounter when using query hints.