Unidentified Flying Object(s) is a catch all term for any object in the air that is unidentified, however along with its acronym UFO, has come to mean spacecraft(s) from or using alien, or extra-terrestrial, technology. The term UFO became popularised in the post-war period when the US Air Force used the term in its literal meaning for in-flight reports. From a security standpoint it made sense for the military to investigate and try to identify what could be potentially enemy craft infiltrating US airspace.
The term 'Flying Saucer' preceded UFO - but because reports were often of objects with different shapes UFO proved a more enduring term. On investigation most UFO sightings are shown to be familiar but often misidentified objects such as birds, planets, other aircraft, clouds, the Moon, insects or even simple dust specks on windows. However many remain unidentified mainly because of the lack of information or because the witness misremembers, exaggerates or simply lies about what was seen.
There have been many famous claimed sightings that excite 'UFOlogists' who sometimes identify the object as alien simply because they are unable to identify it as terrestrial and therefore it must be extra-terrestrial. This of course commits the Argument from Ignorance fallacy by assuming that an unknown cause (Aliens) must be the the cause of something (UFO sighting) for which the cause is otherwise unknown.