Liners can be disposable or reusable and are used next to the baby's skin to catch the contents of the nappy and help protect the nappy itself.
Reusable fleece liners act as a stay dry liner letting the wetness through to the nappy whilst staying dry to the touch and are machine washable. They can be stretched across the toilet and the poo removed and if necessary washed in the flush/with a showerhead. Breastfed poo will tend to be absorbed but will wash effectively in the machine.
Silk liners and silver liners have healing properties and will help nappy rash. Silk liners should be hand washed at 30C.
Paper liners can either be flushable or disposable (depending on type and your plumbing system).