Door frames, casings and linings

[geraldthehamster]

I'm building a porch on the front of my house. I've done the footings, brick to just above DPC level and a floor slab, and am doing the rest in timber frame. My next step is to buy a front door and frame so I can build it in to the studwork as I construct it. I've not done this before, and am wondering what is the difference between door frames, casings and linings here:

http://www.howdens.com/product-range/doors-joinery-collection/frames-casings-linings/

and which I need to buy with my door?

(hope it's Ok to post that link, please remove link if not - it's just to illustrate my question, I'm not plugging Howdens!)

Thanks
Richard