Since pockets are the main dipe of choice at our home, I'm glad to share what I know about some brands/types, (and hope that others can fill in what I miss!)....

Karen explained them well in another post- pockets have two layers that are sewn together- an outer, usually PUL (polyurethane laminated fabric) or fleece, and an inner for keeping baby dry- usually a microfleece or microsuede. There's an opening between the two layers, usually in the back of the waistband, that's used for stuffing an absorbent layer used to soak up pee. No cover is needed with pocket diapers that are made with PUL or fleece outers.

Pocket diapers are quite similar to disposables in ease of use...the absorbent layers can be customized based on when baby wets most during the day, etc.

Once wet in or pooped in, pocket diapers must be washed, as the absorbent layer is inside the diaper (unlike AIOs where the insert can be unsnapped, the cover can dry, and then another insert can be snapped in for use).

Pocket diapers close in a few ways- there are snap and applix (the generic name for Velcro, which is trademarked...also known as hook-and-loop or front touch tape) closures. They can also close in the front or on the side, which is usually a preference based on the way diapers fit your baby. (for example, we've found front closures to be more gentle on our baby's chunky thighs than side snaps, though I've read on other sites that side snaps should allow babies to wear the diaper up to 5# above the diaper size's stated weight limit).

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BabyBlanket Diapers come in PUL outers, and either microfleece or microsuede inners. These are probably the most economical of all the pockets that we own. Made by wahm Wendi (and Rebecca!), they can be found at www.thebabyblanket.com

Doodle Bottoms made by wahm Cory (also a student, she's quite busy these days) come in side snap or front applix closure and are lined in either microfleece or microsuede www.doodlebottoms.com

Ellas Diapers come in PUL and fleece outers, and each one also comes with a hemp insert. They're front applix closure, are usually priced around $15.95 (including the insert), and can be found at www.ellasdiapers.com

First Class Baby Envelopes are made with the pocket opening in front, and come with either PUL or WindPro Fleece outers and microfleece inners. www.fluffymail.com

FuzziBunz have a patent on the fleece-lined pocket diaper concept and are a favorite for nighttime and daytime diapering alike. They're trim, sturdy, and come with a satisfaction guarantee. These are snap front diapers and come in mostly solids, with the occasional buggy print ;)
I've purchased them at several sites:
www.naturalbabies.com
www.lilbunz.com
www.nurturedfamilies.com

Happy Heinys are another great pocket dipe- front applix closure.
I've purchased them at www.lilbunz.com

HarleezPocketz offer front touch tape and side snapping pockets. These have been harder for me to get my hands on, so we don't have any personal experience with them, but they can be found at www.wahmarama.com

Knickernappies are made by wahm (work at home mom) Aherne, and are most like FuzziBunz. The main differences are availability (harder to get), price (generally lower), her prints selection is greater, and the front snaps can overlap, allowing baby to wear one size longer. www.knickernappies.com

Simply Cloth are made by wahm Julie, and are a side snap diaper. Also harder to get (these days), Julie has an incredible selection of prints from which to choose. www.simplycloth.com

Soft Landing Stuffers are either a PUL or fleece outer, and can be made with foldover elastic or turned. Also, they can be front applix or side snap closure. SL diapers can be found at www.softlandingdiapers.com

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Stuffins for pocket diapers can vary greatly and are often chosen based on baby's wetting amounts during the day, parents' preference, etc.

Hemp is made of natural fibers, so the urine is able to disperse more evenly in a hemp soaker.
Examples include:
JoeyBunz Hemparoos (found at www.lilbunz.com and many other sites that sell FuzziBunz pockets)
Stuffers that come with Ellas Pocket dipes
Nighty Newts Soakers and Komodo Dragon Doublers from www.littlelounginglizards.com

Microfiber is softer and less stiff than hemp, but leave urine more concentrated in one place.
Examples include:
MOE Microfiber inserts (found at www.naturalbabies.com and other sites that sell FuzziBunz pockets)
Microfiber towels found in the automotive sections of WalMart, Target, and CostCo (these are great trifolded and stuffed into a diaper- we always use them in pairs)...incidentally, these can tend to bleed in laundry, so it's advisable to wash them alone, with a Shout Color Catcher, on hot water a few times before throwing them in with the rest of your CDing laundry

Prefolds are great for stuffing...they're definitely not as trim as the other options, but offer a great and economical solution for stuffing (especially for those looking for uses for their outgrown newborn sized prefolds). Just trifold (the "wrong" way- horizontally instead of vertically) and stuff into the pocket.

Sorry for the multiple edits...I'm adding as I think of more and as the boys let me ;)