Hummingbirds are the only birds in the world that can fly backwards for a substantial amount of time. This is because the wings' muscle structure is essentially a ball and socket joint, which allows the wings to be moved up, down, backwards and forwards. Not only this, but they can also rotate their wings to form a figure of eight. This is why hummingbirds are also capable of some pretty impressive acrobatic abilities whilst flying.
This is unique to the hummingbird, as most birds have wing structures with powerful muscles that can pull the wings down, with only weaker muscles available to pull the wing back up. This means that the air surrounding the wing is forced backwards, which pushes the bird forward.
Although the hummingbird is the only bird that can fly backwards for a substantial amount of time, birds like herons, egrets, warblers and flycatchers can all fly backwards for a very short distance - this is mainly used as a defensive strategy for each of them.
On extremely windy days, birds like cuckoos can appear to fly backwards, but they are still moving forwards relative to the air.