Fava beans are the dried beans of the Vicia faba species of the legume family. They're the same species as the fresh or frozen green broad beans more familiar in British cooking but fava beans are the fully mature dried fruit of smaller seeded varieties.
Varieties of Vicia faba grown to be eaten as fresh broad beans tend to have larger, flatter, broader (hence the name) seeds. Dried fava beans are also known as field beans, horse beans or even tic beans (a name for the very smallest varieties). In US English however the name fava refers to fresh broad beans, infamously washed down with Italian wine.
While broad beans are picked fresh from the living green bean plants, fava beans have to mature and senesce, the plants and pods dying, drying and blackening, before the beans are harvested.
As the dried seeds of a leguminous plant they're a kind of pulse, like most pulses they can be cooked either as whole beans, skin and all, or as split beans, with the skin removed and the bean split into into its two halves, the cotyledons.