UFOs, (pronounced oo-foes) are things. They are totally real but nobody has any clue what they really are. The are UNIDENTIFIED FLYING OBJECTS, and by definition, nobody can know what they are.
Anyone claiming that they know for a fact that they have identified a UFO as an alien craft has no fucking clue what they are talking about. You cannot identity a UFO, or it is no longer Unidentified, and thus because fucking normal. The most common UFO shape is flying discs, but spheres, pyramids, tetrahedrons, fives, whales, dudes, cloud shapes, blurry lights, and airplane-shaped-UFOs have also been observed. They can be seen against the night sky, or landing on the Moon. They also visit nuclear bases and secret government facilities like Area 51 and Black Mesa Research Facility. They are also hovering over every city worldwide at all times, but just invisible so you don't know.
To wit, not all UFOs are flying saucers, nor or all flying saucers actually UFOs. Sometimes one of those plate-spinning guys drops his dishes all over, and for a brief shining moment before they crash and shatter into dozens of pieces, they are magnificent flying saucers (but are definitely 'identifiable objects').
Some reseachers claim that the term should stand not for "Unidentified Flying Objects", but for "Unidentifiable Flying Objects", as the objects are incable of being defined by our mortal minds. Moreover, as stated above, if they were ever to be identified they would cease to be UFOs immediately. The bulk of them are government craft and aliens from deep space and other alternate dimensions and as such are beyond the pale of human comprehension; even if we did have a reasonable explanation for their existence, UFO researchers would move the goalposts of said understanding and the objects would be inscrutible once again. This is akin to the philosophy behind God, and it means this is hardly a question for science. There are also objects called USOs, or Unidentified Submersible Objects, which are so stupid we don't care to comment on other than to say; of cucking fourse they exist, what's to stop a UFO from going underwater? UFOs are freaking awesome and so is everything underseas so we don't need a damn separate category for them.
Not all UFOs are necessarily craft. Some are large and imported varieties such as IPA-UFOs.