It is hard to really know for certain what these could be. People have reported seeing odd moving lights in the sky for decades, and sometimes many witnesses have reported seeing the same lights from different vantage points. As a scientist, I just do not know what to make of these things since they are never observed by scientists who study such phenomena as 'ball lightning' or the Aurora Borealis. Also, it has been proven beyond reasonable doubt that at least some of these sightings are of bright planets such as Venus or Jupiter. The average person who spends a majority of time indoors at night can be easily surprised by ordinary celestial objects because they are unfamiliar with what they look like from month to month. I am not saying that all reports of 'UFOs' are in this category, but some are. And considering all of the testing that various military organizations do, some of the sightings can even be man-made!
In the last 30 years or so, physicists have studied ball lightning which was once dismissed as folk tales. Now they are still considered odd, but scientists take them seriously. Animals can sense disturbances in the magnetic field near an earthquake fault before an earthquake occurs. And earthquakes can produce light as the crystals in the rock are suddenly compressed and emit what is called piezoelectric light. So, there are plenty of 'odd' natural phenomena that are now respectable science.
The problem with UFO reports is that they are all so SENSATIONALIZED that no self-respecting scientist would want to take all of them seriously for fear of loosing their reputations and their jobs! I am serious about this. There is an enormous stigma attached to anyone that investigates UFOs and ESP. Perhaps if the public were not so convinced that every strange light and every strange atmospheric phenomena were aliens from other worlds coming to visit, scientists could work in peace and quiet to find out what is really going on. Also, the amount of emotion has risen so high in investigating UFOs that no one would believe any evidence that contradicted what they 'truly believe' UFOs are. If scientists prove that a particular observation is either a planet, or the figment of someone's imagination, the scientist would immediately be accused of some cover-up.
UFO reports, like ESP, have evolved into a completely unconfirmable subject because no one wants to believe or trust someone elses 'proof' that some occurrences COULD be hoaxes perpetrated by someone who wants to be famous, or that humans can be easily misled into seeing what they want to believe. This happens all the time in other areas where humans are asked to report what they saw. Just talk to any lawyer about the value of 'eye witnesses' evidence. In many cases it can be unreliable. Now, add the emotions that seem to run rampant when a witness is convinced that what they saw in the sky may have been an alien spacecraft.
As for whether any specific sighting could be a UFO, it is hard to tell since the statements by eye witnesses are too vague at times to be used to eliminate other possibilities. I frequently get notes in the Ask the Astronomer area where people describe something they saw, but fail to tell me:
1) The time of day. 2) What was the atmosphere like? clouds? stormy? 3) What direction did it seem to be moving? 4) How fast did it seem to be moving? 5) What time of year was it? date? 6) Was the shape, sharp or indistinct?With this information, I could eliminate meteors, planets, aurora borealis, and ball lightning, which are the most common UFO misidentifications!