For newborns, you will need newborn sized nappies. One size nappies like pockets only fits from around 3 months. It will still be worth your while to buy newborn nappies as you use between 7 and 12 diapers per day on a newborn up to around 12 weeks. Even if you buy 20 newborn nappies, you will still save a lot of money. You can reuse them for your second baby or sell them on the cloth nappy group on facebook, they sell very quickly, so you won't be wasting money.
As for the terms:
Pocket: a shell lined PUL outer usually lined with microfleece. There is a pocket between the PUL and fleece where you insert the "nappy" which is just a rectangle usually made from micro fiber or bamboo. They are easy to put on, usually have very cute prints or different colours. I love pockets as they clean easily, dry fast and are cost effective, especially if you buy a bulk pack form ALVAbaby.com. I do prefer the bamboo inserts as they absorb more liquid and I usually pair a bamboo with a microfiber insert to ensure that there are no leaks.
Prefold: A rectangular piece of cloth which you fold over twice to form a long insert. You use these with a separate PUL waterproof cover.
Flats: A terry square or a muslin square folded into a nappy and snapped on. You can also use old receiving blankets to make your own flats. This is the cheapest option but not many daddies are willing to try putting one on. You also need a waterproof cover over these.
Fitteds: A all in one nappy made for a certain size baby. Great for newborns, and great for night nappies. You usually put a waterproof PUL cover over these for nights.
Hybrid: Basically the same as a pocket, but instead of putting the insert into a pocket, you snap it inside a cover. See video here:
www.gro-via.com/learn-cloth-diapers.html