Support for delta replication was introduced in SMS 2003.  Delta replication means that when a package source files are updated, only the files that have been updated will be replicated to the distribution points.  This results in significant bandwidth savings.  With the release of SCCM 2007 we get even more granular.  Now, we are able to detect the changes down to binary differences WITHIN a file and replicate just those differences when updating a distribution point.  As an example, assume you need to update a 10 MB file in your package source.  With delta replication that file would be copied to all of your distribution points since it was updated - but the change may have only been 1.5 KB worth of data.  WIth binary delta replication we are able to detect just the 1.5 KB of data that has changed and send just that amount of data to update distribution points.  A few points about binary delta replication

-Binary delta relication detects the 'chunks' of files that have changed and updates just that data
-Binary delta replication is fully supported to all distribution types, including branch DP's
-Binary delta replication is only used to update DP's and is not used by clients when downloading information from DP's