A wet bag is a small bag usually used for wet/dirty diapers in the diaper bag. This is handy to have for outings.
A hanging diaper pail is ideal for small rooms where there's no space for a plastic pail. These can also be used as pail liners for plastic pails.
A pail liner is used inside of a diaper pail or trash can.
I recommend buying both a wet bag and a pail liner or hanging diaper pail. All of these are waterproof and can be washed with the diapers.