You have come to the right place
Ok this is what I have learned.
1st off you need GiGi to come in this thread, Im sure she will at some point.

Now what I can tell you.
Hemp is absorbant
Bamboo is absorbant
Wool is for covers
PUL is another material used for covers. Its like a plastic but its not, it breathes
Malden mills is a fleece often used as covers as well.
Prefolds are like your old flats but have a absorbant layers in the middle. So its like having your flat nappy "prefolded" to where you need the absorbant bits.
Fitted nappies need a cover.
AIO (all in one) is just that, a fitted nappy with a built in waterproof layer. so thats it just grab your AIO & put it on, job done. But can take longer to dry.
POcket Nappies are a nappy made of two layers with a opening at one end so you can add your absorbant material inside it. these are good so you can make the nappy as absorbant as you need it but then can pull it all apart for faster drying. The outter layer is usually waterproof so no cover is needed.
Longies/Shorties & soakers- These are all wool covers. Longies are long pants, Shorties are shorts & soakers are like pull on undies. You lanolise them & you have a fantastic water proof cover that breaths & keeps bub warm in winter & cool in summer.

My Favs are a nice fitted made from hemp or bamboo teamed with a wool cover (longies/shorties etc)

eta - seems like a lot to take in seems there are so many different MCN on the market but really its pretty simple once you get into it. I have only been using cloth since June. Now I am using them full time & making my own.