SQL Server organizes the memory it allocates into two distinct regions: the BPool (buffer pool) and MemToLeave (memory to leave) regions. If you make use of AWE memory, there's actually a third region: the physical memory above 3GB made available by Windows' AWE support. (Refer to Chapter 4 for details on AWE.)
The BPool is the preeminent region of the three. It is SQL Server's primary allocation pool. MemToLeave consists of the virtual memory space within the user mode address space that is not used by the BPool. The AWE memory above 3GB functions as an extension of the BPool and provides additional space for caching data and index pages.