Microsoft offers two mechanisms for tracking changes:  Change Data Capture (CDC) and Change Tracking.    CDC is an asynchronous process which captures the exposed data from the Transaction log.  It not only captures the fact that data changed but it captures the actual data that changed.  It uses the same log reader as the Transactional Replication. On the other hand, the Change Tracking feature is a lightweight, synchronous process which captures only the fact that data was changed.  It will not capture the actual data that changed.  These net-changes are available for query.   It is analogous to the change tracking performed by Merge Replication.

More info can be found at:

http://msdn.microsoft.com/en-us/library/dd266396.aspx

http://msdn.microsoft.com/en-us/library/bb964713.aspx