Storage for DBAs: Data deduplication - or "dedupe" - is a technology which falls under the umbrella of compression, i.e. reducing the amount of capacity required to store data. In very simple terms it involves looking for repeating patterns and replacing them with a marker: as long as the marker requires less space than the pattern it replaces, you have achieved a reduction in capacity.
One of the most interesting articles explaining De-duplication from a DBA's perspective.