The most generic definition of an Unidentified Flying Object, or UFO, is any flying object or phenomenon that cannot be identified by the observer. Various studies show that after investigation, the majority of UFOs are usually identified, and are relabeled IFOs or Identified Flying Objects. Therefore, some stricter definitions reserve the label "UFO" for only those instances where the objects remain unexplained after a proper investigation.

The percentages of IFOs vs. UFOs varies with the researchers, study, and case sample, ranging from only 5% to 10% being UFOs, according to The J. Allen Hynek Center for UFO Studies, to 20% to 30% being UFOs according to earlier U.S. Air Force statistics or the later Condon Committee UFOs have been spotted in many different places around the world.

Reports of unusual aerial phenomena date back to ancient times, but modern reports and first official investigations began during World War II with sightings of so-called foo fighters by Allied airplane crews and in 1946 with widespread sightings of European "ghost rockets." UFO reports became even more common after the first widely publicized United States UFO sighting, by private pilot Kenneth Arnold in the summer of 1947.

Many tens of thousands of UFO reports have since been made worldwide. But many sightings may yet remain unreported, due to fear of public ridicule because of the social stigma surrounding the subject of UFOs, and because most nations lack any officially sanctioned authority to receive and evaluate UFO reports.

Source: http://en.wikipedia.org/wiki/UFO