These are strips of fabric (usually fleece or some type of rice-paper) that lay on top of the inside of the diaper. It will be the material that actually touches your baby's skin. Its purpose is to both wick away moisture (or draw moisture away) from your baby's skin and to make cleaning up poopy diapers easier. It can also be a safeguard if you want to protect your diapers from certain ointments (prescription creams, etc) Note: They do not add any absorbency.
When it wicks away moisture, it obviously leave the baby's skin feeling dry. This means more comfort to the baby, less skin irritation, and more protection from diaper rash.
When you are on the go, having the disposable liners (rice-paper like material) make cleaning up poopy diapers a breeze. Instead of carrying around a poopy diaper in your wet bag, you just remove the liner and flush it down the toilet. Moreover, you aren't dunking your hands or swishing the diaper in a public restroom. Yuck! These disposable liners also wick away moisture and are very inexpensive, so it's a win win.
The fleece liner serves the same purpose as the disposable liners. The fleece liners are obviously washable, so you can use them again and again. I would also bet that they are more comfortable against the skin compared to their disposable counterparts. Fleece liners are a great option to add more comfort to babies exclusively wearing prefolds or other cotton materials leaving their skin damp.
Ok, diapers inserts. They are multiple layers of material (cotton, hemp, bamboo, microfiber, etc) sewn together that are design to absorb all the liquid in the diaper. Most often they are inserted into a pocket-style diaper but sometimes can also be laid inside a diaper cover. I typically don't recommend laying it inside a diaper cover because it can move around causing the diaper to leak or sometimes sag down. And putting a microfiber insert directly against the baby's skin can cause a rash and is highly not recommended!
Jen Starks, Owner