First, a wool diaper cover is thermal - "it can store water vapour up to 35 per cent of its own dry weight yet it remains dry to touch and speeds up the body's own cooling system." (2) To attest to this fact, wool is most regularly suggested for night-time usage when leaks most often occur in abundance, although certainly can extend to everyday wear. Second, while absorbent, they still remain breathable, allowing for a maximum amount of circulation around baby's bum. This helps prevent diaper rash, but also alleviates the health concerns of trapped heat within a diapering system. Finally, wool contains natural lanolin which creates a natural waterproof barrier or repellancy. (From The Diaper Hyena, Diaper Covers: Why Wool's Cool)(For more information about the amazing qualities of wool and why to use them in cloth diapering, read "Why Use Wool for Diaper Covers? Wool Science for Cloth Diapering Parents" by Marc Pehkonen!
I've made several wool diaper covers for my babes and love them, especially to prevent leaks at night. By felting, sewing and "lanolizing" old thrift store sweaters, wool covers can be affordable and fun. (Here's one pattern I've used.)
You can find more info on cloth diaper websites and discussion forums!
Here's one of my first attempts on baby Moses:
...but I love this newest creation!