What is Corona & Which programming language is used to develop an application in corona?
What physics Engine is corona using?
and any other information that is absent in their site?
You write code in Lua when developing in CoronaSDK. That information is definitely on their site, although it's not as prominent as it used to be (I haven't looked at the front page in a while).
For example, here's the page under the "Games" heading: http://www.anscamobile.com/corona/games/
First sentence: "Built on top of OpenGL, OpenAL, Box2D, and Lua"