Modern nappy covers are used over fitted or flat nappies and are made of water-resistant and breathable fabrics. These include PUL, wool or fleece.

PUL stands for polyurethrane laminate.
It is a film applied to the back of 100% polyester fabric to allow evaporation of moisture through the fabric while allowing for water resistance.
PUL covers are machine washable, trim and economical.

Wool has been used for centuries as an effective nappy cover. Its unique properties include absorbing up to 30% of its weight in moisture without feeling wet, and odour-resistant properties. Very breathable fabric.

Though it is a handwash only item, it requires minimal maintenance every 2 weeks to a month.

Fleece is the modern day equivalent of wool in terms of its breathability. However it does tend to retain smells so requires regular laundering compared to wool. It is generally machine washable.