Woks, skillets, sauce pans and grill pans are often made of metals prone to rusting, in particular cast iron and carbon steel. Those metals are also porous, and prone to sticking. By seasoning cookware made from these metals we protect the cookware from rust and create a virtually non-stick surface. Cookware made from other metals are sometimes seasoned too.
Seasoning happens naturally over time and continued use, but it can also be done by deliberate methods. Most commonly thin films of oil are baked on to the surface of the cookware to create a hard, polymerized layer that clings to the metal surface. This smooths over the metals natural pores, causing the non-stick surface, and creates an impenetrable layer to protect the cookware from rust.
Questions encompassed by this tag will concern methods of creating and maintaining a seasoning layer on all kinds of cookware.