U.F.O. or Unidentified Flying Object is the term given to an object that is seen in the sky that can't be explained. An alien explanation is not always given. An optical illusion might cause a plane or a bird for example to not look like what its supposed to. The most famous film to base itself around U.F.O. is Close Encounters of the Third Kind which came out in the late seventies which tells the story of human contact with aliens after sightings of strange lights in the sky. When the film came out, the number of UFO sightings shot up. The UK used to have a dedicated team of investigators that used to look into UFO sighting but that team was closed down in 2009 because it had no defence purpose. Ref: B.B.C.
It is not just the general population that reports UFO sightings. People in authority have been known to make UFO sighting such as pilots who reported a near miss when they were near Heathrow, London's main airport Ref: Telegraph
Before James Carter became President of the United States, he was Governor of Georgia and during that time he filed two UFO reports. Ref: UFO Evidence
There are people who believe that UFOs are from other world but those sceptics ask in response why would they travel so far across the galaxy or even further and then turn round without making contact. The response back is that the aliens don't see us as worthy because we have not achieved the required level of intelligence to make first contact, something that was a requirement for Vulcans in Star Trek before they introduced themselves to us.
The following video is from N.A.S.A. which shows a strange object in on a space feed but then all of a sudden there's technical problems and the live feed went down. I'm not be alarmist or theorist, just showing what is been seen by many people and no explanation given.
Could the mysterious object be the X-37B shuttle which was in orbit round the Earth for two years and no one knew anything about it until it landed at Vandenberg Air Force Base in Southern California ?
Roswell, New Mexico is the scene of one of the most famous UFO sightings in the United States when an alleged UFO which the United States Air Force state was a weather balloon that crashed. The Conspiracy Theorists will not let the issue lie. There have been untold number of books and documentaries on the subject. There has even been a television series set in Roswell about a group of aliens who were left behind on Earth.
In the United Kingdom, their most infamous incident was rendlesham forest incident which prompted Lord Hill-Norton, a former chief of defence staff to write to Michael Heseltine, the then Defence Secretary at the time where it was later discarded, the incident that is. Ref: Telegraph
Some theorize that the meteorite that crashed in Tunguska may have actually been an UFO by the way the area was so devastated. Investigators from Moscow didn't arrive on the scene until years after the actual incident.
The Great Pyramids and the Sphinx is said by some to have been influence by a meeting from Aliens who travelled here in UFOs. The theory is the primary basis behind Stargate franchise.
It is not a recent phenomena, of the earliest UFO sighting according to took place in 214 B.C.E. when Titus Livius Patavinus (Better known as Livy) wrote in his diary that he saw strange portents in the sky. However as communications have been getting better so have recording UFO sightings. Ref: UKTV