What does /mnt/asec directory contain? What does /mnt/asec directory contain in Android? Can someone explain in brief?
Thought Google would have an official definition on Android Developers, but surprisingly not. So with a lil bit of Google I dug up this: 

Android Security Internals: An In-Depth Guide to Android's Security Architecture

Look around P80 for related content. From what I understand, ASEC (Android Secure External Cache) was implemented in a time when Android devices come with limited storage and has to resort to external storage (i.e. SD card) for apps, and its purpose is to securely store part of the apps so as to prevent corruption and piracy by copying SD card contents.

Wikipedia also has a bit of intro here.