A fitted diaper is a diaper that does not have a waterproof cover on it. with the VB pattern there is a soaker that you tuck in next to the baby. Some people make the soakers snap in while others just lay them in. You would need to make covers.
a pocket diaper is a waterproof cover with a pocket in it to stuff with something absorbant. The pocket part is made of a fabric that allows the urine to "wick" through to the absorbant layer.
an AIO is a diaper that is all in one..cover and absorbant layer all together.. They take longer to dry typically.
The main reason people would go with a pocket diaper over an AIO is drying time.
If you have other questions feel free to ask away and if I know I'll answer.

Cristy