The upper, usually made of leather or a combination of fabric and leather.
Leather is the most durable, form fitting, and waterproof material available.
Full-grain is the outside layer from the cow's hide. This grain is most durable against abrasion and water penetration. Any layer that is not from the outside layer of the hide is considered to be a split-grain. Split-grain layers are softer, breathe better, and stretch easily. You can waterproof your boots by treating them with Nikwax.