The term Unidentified Flying Object (UFO) refers to any object, which is flying and cannot be identified. This can be anything you can imagine, from planes seen at funny angles to secret experimental aircraft. There is an endless list of things which when seen by the untrained eye will not be readily identifiable. Most of the time, reported UFOs are in fact perfectly identifiable objects (let's say that 90 to 95% of the time). Most UFOs are not UFOs at all.

But these do not explain everything. There are some aircraft which fly into the airspace of countries all across the world, they are tracked on radar and are even sometimes chased by the military aircraft. The thing that sets these aircraft apart is their characteristics; these are unusual shapes such as black triangles, glowing orbs, cigar, etc. These craft can do some decidedly odd things, they can hover silently, and they can do seemingly impossible maneuvers and whiz off at incredible speeds. When they are tracked on radar they have been found to be doing acceleration which no human could survive. That is of course when they can be tracked, most of the time they cannot be tracked on radar due to what is presumably some sort of stealth type technology - this must be very advanced stealth technology as none of the current publicly known stealth aircraft are completely radar invisible.

The other explanation for UFOs is of course that they are no from this world at all but from a different planet. This cannot have a scientific explanation yet, and according to the Physics theories, they cannot exist. To learn more about UFOs, please go to the next interesting web site.
UFO's in NASA