I'll try to break it down for you here, but as always, please ask questions. I'll do my best to give a helpful answer!
Consider this a bit of a 'glossary,' but with descriptions and images.
Flats:
A large {typically} cotton square of single-layer cloth that you fold to fit your baby's body. Flats are very easy to launder, dry quickly, and aren't prone to 'stinkies' or other trouble that you might have read about. On the other hand, flats can be a bit overwhelming for a new family, as they require some additional folding and attention at diaper change time. Flats can be held in place with pins, a product called a Snappi, or a number of other contraptions available on the market. You can also wear flats without any fasteners, because of advancements in the function of covers.
Prefolds:
A sized, multi-layer {usually} cotton diaper. They're rectangular in shape, and what most people picture when they think of cloth diapers. Prefolds can be held in place with pins, a Snappi, or a number of other contraptions. You can also wear a prefold without any fasteners, like flats, because of advancements in the function of covers. Prefolds are long-lasting and easy to care for, and tend to resist odors and things more effectively than some other options. Like flats, there is a touch more work involved at change time, but not much.
Fitteds:
A completely absorbent diaper that fits, well, like a diaper! Fitteds may come in sized or one-size options, and the materials vary by company. Some come with velcro or snap closures, and others require the use of pins or a Snappi. No waterproof barrier is included with a fitted, so a cover becomes necessary. Fitteds are a great nighttime option, as they provide a lot of absorbency without a ton of bulk.
Covers:
No absorbency is provided by a cover. Think of the old fashioned 'plastic pants,' but re-designed to fit snugly and comfortably, while offering both breathe-ability and a waterproof barrier between baby's diaper and the outside world. Covers are necessary over flats, prefolds, and fitted diapers, and may often be used for more than one diaper change per day.
All-in-Ones {AIO}:
As the name suggests, you get all you need, in one. An absorbent inner with a waterproof outer. AIO's are notorious for being tougher to launder. Though this isn't always the case, the stitched-together nature of the diapers can make for a bit more effort in the laundry room. On the other hand, AIO's are easy to put on and often a favorite for their simplicity at change time. An entire AIO must be laundered between uses.
All-in-Twos {AI2}:
Almost identical to an AIO, except that the soaker {the portion designed to absorb baby's urine} is typically removable, or may have options for adding/taking away extra absorbency.
Pockets:
Again, as their name suggests, pocket diapers have... a pocket! Usually located at the back waistband area of a diaper is a pocket into which you place an absorbent soaker or insert. The outer layer of a pocket is a waterproof barrier, and the inner layer is made of a stay-dry material designed to pull moisture away from baby's skin and into the soaker/insert. Pockets may be stuffed ahead of time for simplicity at change time, making them function like an AIO. Upon removing a diaper, you'll need to remove the inserts from the pockets before placing them in the diaper pail. Both the outer portion of the diaper {the pocket, or shell} and the insert need to be replaced each time a diaper is changed.
Hybrids:
There are many 'hybrid' systems available on the market now. They typically consist of a shell {much like a cover}, and then some sort of specially-designed soaker/insert. The inserts may snap in to the shell, or may simply lay inside the shell, and there is often a disposable insert available as well. Like a cover, shells are often usable for more than one diaper change at a time, and the disposable inserts can be a great option for travelling or that unplanned washing machine break-down.
Shells, Soakers, Doublers, Liners:
A shell is another name for a cover. It's the outer portion of the diaper that keeps mess and wetness contained.
A soaker is the absorbent portion of a diaper. In a pocket diaper, the soaker is stuffed into a pocket. With a hybrid system, the soaker may snap or lay inside of a shell or cover, and so on.
Doublers are designed to up your absorbency when you need more than what a diaper provides. They can be made from a variety of materials, and may go against baby's skin, or under something else, such as a prefold.
Liners don't provide any absorbency. Instead, they act as a barrier. Typically made of fleece, a liner acts to keep rash cream off of the surface of your diapers, or to provide a stay-dry quality {fleece is moisture-wicking} in a diaper that doesn't have its own stay-dry layer, such as a prefold.
Final Thoughts:
As with any product, there are differences and variations from one company to the next, but this should give you a jumping-off point if you're new to cloth.
The most cost-effective diapers on this list are flats and prefolds, used with covers. Hybrid systems tend to fall in the middle-range for cost, followed by pockets and AIO's/AI2's. Finally, fitteds are generally on the pricier end of the spectrum.
Watch for another post soon about cloth diapering accessories!
{thanks to Diaper Lab for use of the images!}
