A nest and a roost are two very different things, although both are used by birds, and in some cases, birds use both, depending on the situation and their needs. Simply put, a nest is a constructed space in which birds can sit or squat, while a roost is a perch, which a bird must grip to sit on. Nests and roosts are both used for sleeping and sheltering from predators, but only a nest can be used to incubate eggs.
When birds build a nest, they collect materials to create a structure and line it, or to line an existing space. Depending on the species, these materials can include twigs, feathers, grasses, animal hair, paper scraps, and a wide assortment of other things. Many nests are constructed in sheltered, hidden areas, allowing birds to hide on the nest. This is extremely important, since nests are used for incubating eggs.
When birds breed and produce eggs, the eggs need to be kept warm until they hatch. This usually means that one or both parents must sit on the eggs for a set period of time, allowing the embryos to develop into baby birds. Obviously, eggs cannot balance on a perch, so a nest needs to be built to hold them during the incubation period and to protect the young birds until they are old enough to fly.
A roost, on the other hand, is simply a perch. Many birds roost to sleep, flying up into trees and perching on twigs and branches to shelter themselves from predators on the ground. Birds also hang out on roosts to keep an eye on potential prey and to lounge out of reach of predators. Often, a nest and a roost are very close, providing options for birds while they breed and raise young.
Another crucial difference between a nest and a roost is that while most birds build nests of varying types, not all birds roost. Only passerine birds can roost, as they have specifically developed feet that allow them to grip things. Passerines have three forward facing toes and one toe that faces in the other direction, creating a clamp that can be used to keep the bird on a branch, even while asleep, while other birds lack this ability.