Web-footed describes an animal whose toes are joined together by skin, so the foot works like a paddle. Ducks, geese, swans, frogs, penguins, otters — all web-footed. The skin between the toes lets them push water and swim fast, which is exactly why the animal has it.

The 'web' here is not the internet and not really a spider's web either — it is the older meaning of web: a thin flat sheet of something stretched across a space. That is the same picture in all three uses, which is worth noticing once and then never forgetting.

It is a describing word that goes before a noun almost every time: a web-footed bird, web-footed animals. You would rarely say 'that duck is web-footed' — you would say 'ducks have webbed feet'. That is the more natural everyday sentence, and it uses the sister word 'webbed'.

In writing it usually keeps the hyphen. You will also see 'web-toed' occasionally, and doctors use 'webbed' for the same thing in humans, where two fingers or toes are joined at birth.