“So what is it? A memory mapped file allows you to reserve a region of address space and commit physical storage to a region (hmmm, sounds like virtual memory, isn’t it?) but the main difference is that the physical storage comes from a file that is already on the disk instead of the memory manager. I will say that it has two main purposes:

• It is ideal to access a data file on disk without performing file I/O operations and from buffering the file’s content. This works great when you deal with large data files.
• You can use memory mapped files to allow multiple processes running on the same machine to share data with each other.“

OMG! You can even specifiy views on a memory mapped file… from different processes… .NET 4 FTW!

Source: http://blogs.msdn.com/salvapatuel/archive/2009/06/08/working-with-memory-mapped-files-in-net-4.aspx