There are several baby diaper cover fabrics commonly used today. Each type of fabric has its own features to consider.
Cloth diaper covers (the waterproof layer of your cloth diaper system) are generally made of one of the following fabrics:
Polyurethane laminate diaper covers are made from a film that is bonded to fabric. The film is what makes the fabric waterproof. PUL baby diaper covers are typically somewhat shiny, and they vary in thickness and feel from brand to brand. This type of cloth diaper cover is very commonly used, durable and relatively inexpensive.
You may see the term 1 MIL or 2 MIL used in reference to PUL cloth diaper covers. These terms refer to the thickness of the laminate layer that is bonded to the fabric (1 MIL is thinner, 2 MIL is thicker). Don't assume that thicker is automatically better.
While some parents prefer 2 MIL PUL diaper wraps because they find them more water resistant, I personally prefer the 1 MIL Thirsties that I use over my 2 MIL PUL diaper wraps. I find the 1 MIL PUL covers to be just as water resistant as the 1 MIL covers, and they are less stiff and more pliable, like fabric, which makes diaper changes easier.
Wool diaper covers are naturally antibacterial and water resistant. Wool has a wicking action, so it will draw moisture away from your baby. It is very breathable and will keep your baby warm when it's cold out, and cool when it's hot. It is very absorbent and although wool requires special care when washing, wool diaper covers do not require frequent washing.
Fleece is extremely soft and provides great airflow. It wicks moisture, so it can help keep your baby's skin dry, but it can also feel damp on the outside if diapers are changed infrequently.
Vinyl diaper covers do not provide good airflow. They trap heat within the diaper, and they provide no wicking action, so they also trap moisture near your baby's skin. Vinyl baby diaper covers can contain harmful phthalates. They are inexpensive and widely available in stores; however, they are not a diaper cover that I would personally recommend.
In addition to choosing the type of fabric that best suits your needs, you'll also want to make some decisions about the style of baby diaper cover you'd like to use. The article Cloth Diaper Covers provides an overview of cloth diaper cover styles.