Why do you not mention the fabric this is made out of and what type of seams it has?

These fountian covers come with bottom-tie nylon drawstrings for a snug and secure fit. They are made of durable, reinforced polyethylene, that is waterproof and UV stabilized to be used year after year.

Is this cover good for winter purposes to keep water and snow out.

Correct! The fountain covers are designed to be waterproof to help keep moisture out of your fountain. It is very important to winterize your fountain. You can view more about winterizing your fountain here.