A catwalk is a very narrow, high bridge. You might see workers on a catwalk during the construction of a skyscraper, or you might see a model strutting her stuff on a different type of catwalk.
Any high, skinny bridge can be described as a catwalk, but it's more likely to refer to an industrial walkway, like one in a factory or other workplace. The kind of catwalk on which a model struts is different — it's lower, not so precarious, and often extends into the audience. The original catwalks, in the 1880s, were on ships and backstage in theaters, and they were named for the careful, delicate way a cat walks.