Storing BLOBs Externally in SharePoint 2007
Documents in SharePoint Document Libraries are stored in the SQL Server database. In most situations this works well – backing up the SQL Server content database backs up all the documents in a consistent way.
However, for storing Binary Large Objects (BLOBs) may be inefficient and costly. It may be more efficient to store such BLOBs in the file system and then link these files to SharePoint Document Libraries.
This can now be done through the ISPExternalBinaryProvider interface. Prior to SharePoint 2007 SP1 this component was available as a hot fix, but is now included mainstream.
The interface is documented here: http://msdn2.microsoft.com/en-us/library/bb802976.aspx.