Much data is unstructured, such as text documents, images, and videos.
This unstructured data is often stored outside the database, separate from its structured data.
This separation can cause data management complexities. Or, if the data is associated with structured storage,
the file streaming capabilities and performance can be limited.

FILESTREAM integrates the SQL Server Database Engine with an NTFS file system by storing
varbinary(max) binary large object (BLOB) data as files on the file system.
Transact-SQL statements can insert, update, query, search, and back up FILESTREAM data.
Win32 file system interfaces provide streaming access to the data.

FILESTREAM uses the NT system cache for caching file data. This helps reduce any effect that FILESTREAM data
might have on Database Engine performance. The SQL Server buffer pool is not used; therefore,
this memory is available for query processing.

FILESTREAM data is not encrypted even when transparent data encryption is enabled.

To read more, go to:

http://technet.microsoft.com/en-us/library/bb933993.aspx