Waht is the difference between pattern Planes & Aerobatic Plane ?

I would think that a pattern plane would be one that is a purpose built plane Non scale to be flown in the F3a pattern class. An aerobatic plane is anything that would simulate a scale aerobatic plane. All planes are aerobatic to some degree. But for general discussion I would say thats how I interpret it.

What is the difference between pattern Planes & Aerobatic Plane ?

Pattern Planes are aerobatic planes built to perform a specific set of manuvers (aka the "pattern"). the international designation is F3A. Check out the website for additional information.
http://www.nsrca.org/

Aerobatic Planes people are generally referring to are scale versions of full size planes. (I don't like the term "real" when referring to a full size plane. Scale planes are smaller and just as "real". Put together a 40% scale model and the money invested is far from imaginary). Check out the IMAC website for additional information.
http://www.mini-iac.com/main.asp

Also check out the AMA website for the different categorys and rules differences.
http://modelaircraft.org/templates/ama/

Hope this helps,
Bill