I can write pages and pages on this software and i would still not get across what I intend to teach you about it.
If you are working with a "digimat" artist or you are trying to figure out how to give your low budget project a professional three dimensional feel and look while working in blue screen. This a short answer, with long term success and save time in rendering.
Wiki definition of NUKE by The Foundry http://en.wikipedia.org/wiki/Nuke_%28software%29
Here is the best explanation and definition of what it takes to get 300f of video gold https://youtu.be/VMgDcTO9XiE
This is by no means a complete answer to the question but it shows you how to get it done
Oh! By the way did I tell you that The Foundry has made a non-commercial version of NUKE available to you for free to help you get your legs under you http://www.thefoundry.co.uk/products/nuke/non-commercial/
The more you know the less you will be confused about how it works. I will admit this the more you look behind the curtain to know how the magic is done you will be less amazed.