Yes Mechanical Desktop (MDT) and AutoCAD Mechanical (ACM) are very different products.
MDT was an earlier generation 3D parametric, feature based modeling product (based on AutoCAD) that has been replaced by Inventor; all support for MDT 2009 (the last release) ended when Inventor 2013 began shipping. ACM is a mechanical design optimized version of AutoCAD that's an ongoing, main stream product.
I know that the differences can be confusing, but if you understand the difference between ACM and Inventor, basically the same is true between ACM and MDT.
I hope this helps.
Roger Mollon Sr. Director, Software Development Design, Lifecycle, & Simulation Product Group Autodesk, Inc.