terms

Hey everyone, I am extremely interested in finance and have started to read up on it daily. I am quite early in dealing with all this so please overlook my inexperience.

I have an internship and a business development firm that works with investment bankers, mergers and acquisitions, and the like. But I have come across a few terms that I just can't completely understand. I have been financial dictionary.com or whatever but still am confused. Can someone here please give me a more comprehensive and better definition of these terms:

-mortgage backed securities
-reverse mergers
-public shells

any feedback is greatly appreciated!!