I don't know which one to use. which is the best one actually ??? Or else, are both of them the same ???
In some article, I read that simpleCV is sort of an interface which gives you access to openCV. I don't understand that point. Then why do we install openCV separately ??
OpenCV is a library that can be used with tons of different languages (C, C++, Java, Python, etc.). It provides standard things such as image capture, image manipulation, etc.
SimpleCV on the other hand is a framework including several libraries (as far as I know not only OpenCV) and uses Python for scripting. Due to the nature of Python, you can either run scripts or use an interactive shell to do computer vision stuff and related tasks.
Which one to choose? This really depends on your usage scenario. For quick prototyping I'd guess SimpleCV is far superior, but for actual implementation/usage, OpenCV offers a lot more possibilities (although at a higher complexity; e.g. being able to be included in native applications as well as embedded systems).