I own Deep Shredder 12 UCI, if I remember I believe that's the most various and best of the bunch with a few more features, UCI if you didn't know stands for Universal Chess Interface.
According to the store here, Deep Shredder UCI uses multiple cores, which makes sense why I own it since I have a i7-2600k which supports 4 cores and a virtual 8.
As EAPidgeon mentioned, the Shredder engine can only use one core, while the Deep Shredder engine can use multiple cores.
The Shredder versions available from the Shredder web page come with the Shredder graphical interface, while the Shredder versions available from the Chessbase webpage come with the Fritz 12 graphical interface.
The Shredder Classic 4 program from the Shredder webpage lacks some of the features offered in the Shredder 12 interface; The biggest difference is that the Shredder Classic 4 engine is a couple of hundred elo points weaker than the Shredder 12 engine. Other differences between the two can be seen at this link:
http://www.shredderchess.com/chess-info/features/shredder-versions-comparison-chart.html
So I am thinking of getting Shredder for PC, and I have checked Shredder's official webpage at http://www.shredderchess.com/pc.html . There it lists three variants: Deep Shredder 12 UCI, Shredder 12 UCI and Shredder Classic 4. The page at http://chessbase-shop.com/en/categories/112 lists another two: Shredder 12 and Deep Shredder 12.
I don't know what the difference between these variants is and I am asking for help. Can anyone share thoughts? Thanks.