ADE is basically the legal key to unlock purchased or borrowed epubs so they can be read.
Information about ADE:
http://www.adobe.com/products/digitaleditions/
I'm sure others more knowledgeable than I will chime in, but I do think ADE is usually sufficient for most purchases. (I pretty much exclusively download from the public library for free. I have only purchased 2 books so far in over a year of ownership of my ereader: a pdf manual for my camera that resides on my desktop and 1 epub)