SQL Server uses bitmaps internally to speed up query execution by eliminating rows with key values that cannot produce any join records. SQL Server uses these bitmaps only in parallel query plans. Bitmap is a physical operator.
Graphical execution plan icon
How to: Display an Actual Execution Plan
Bitmap Create Showplan Operator
Logical and Physical Operators Reference
Displaying Execution Plans by Using the Showplan SET Options (Transact-SQL)
Parallel Query Processing
Getting SQL Server 2005 Assistance