Recently I've been searching around about Unity and found out that it is really a great game development tool because it provides a lot of features which can simplified your works as you developing games.
I would say it is almost like a game engine because it handles collision, physics, audio, and rendering. Besides that, it does provide scripting ability (C# and Javascript) which allows you to code your own behaviors and apply it on your games. Cool, isn't it?
In terms of publication, it supports almost all platforms like Windows, Macintosh, iPhone, Android, Wii, PS3, and Xbox 360. The only thing which doesn't support is Open-Source OS like Linux. There are 2 versions of Unity which is Unity Standard and Unity Pro. For the product features comparison you can visit the following link:
It is not a tool just to develop some normal casual games but it can be developed for AAA games. Although it may not be as powerful as Unreal Engine but I believe Unity might be catching up in future!
Finally, I would like to say:
I Love Unity!