What is the difference between inserts, doublers, and liners?

Inserts - Typically include at least 3 layers. The insert is the main absorbant layer of a pocket diaper. Inserts can also be added to prefolds, fitted diapers, and all-in-one diapers for an extra layer of protection.
Doublers - Usually include 2 layers. Doublers only boost existing absorbancy. They are not absorbant enough to be used by themselves. They're trimmer than an insert so they aren't as bulky when used with a prefold, fitted, or all-in-one. These can also be added to a pocket diaper.
Liners - Usually include 2 layers. Doublers only boost existing absorbancy. They are not absorbant enough to be used by themselves. They're trimmer than an insert so they aren't as bulky when used with a prefold, fitted, or all-in-one. These can also be added to a pocket diaper.