“What’s the difference, don’t they all just soak up pee?”
Well yes- they all soak up pee. But they also all do it pretty differently and in different amounts. As a general note– anything with a fleece top is going to help your baby stay feeling dry, which is important to a lot of parents. If a diaper or insert isn’t fleece topped, you can always add a fleece liner of your own. They can be bought premade, or you can make your own out of a 100% polyester fleece blanket. Just cut to size, no sewing required!
Microfiber (commonly abbreviated MF) is a very common material in cloth diapers, mostly seen in AIOs and Pocket inserts. It is the cheapest of all the materials available. It is very quick to absorb, but it doesn’t hold as much as other materials. It is also prone to compression leaks. This means when pressure is put on it (like your child sitting in a car seat, or wearing tight pants), liquid tends to squeeze back out and leak. Most all of the microfiber you will see in an AIO has a fleece top on it, which helps to keep the baby feeling dry. A lot of pockets will come with a microfiber insert, which is not fleece topped. This type of microfiber must not be placed directly against skin. It needs to be wrapped in another item (like a flat) or stuffed in a pocket.
Cotton is another common material. It comes in many varieties, and is also used to blend with other natural materials to keep them feeling soft.It is more absorbent than microfiber, and locks in liquid better as well. Flour sack towels are your cheapest cotton option. There are 2 varieties of cotton for making actual diapers: bleached and unbleached. Bleached cotton has been treated to remove its natural oils so that it is ready to absorb liquid. Unbleached cotton has not been treated, and you will need to wash it a few times to strip the oils out of it so that it will be fully absorbent. An easy way to tell the difference is the color. Bleached cotton is very white, and unbleached cotton has a yellow tint to it.
Bamboo is natural(ish), but has been heavily processed to be made into absorbent material. It’s technically Rayon. It’s more absorbent than MF or cotton, and locks in liquid well just like cotton. It tends to be less trim than most cotton options out there, however. Many fitted diapers are made of bamboo or a bamboo blend, because of how absorbent yet soft it is- it feels a lot like a nice soft towel.
Hemp is the king of absorbency. It is also super trim, you don’t need much hemp to absorb a lot of liquid, because it is so dense. It also tends to not be very soft- so it is often blended with cotton. There are many different blends out there, but the higher the hemp content, the more absorbent the material. High content or pure hemp is also really slow to absorb, so it is often paired with a faster absorbing material, like cotton or MF.
There are of course some interesting blends out there that I may not have mentioned here, but just know these 4 basic materials and you should be able to judge how a particular insert or diaper will work for you!