I think the most difference between science and pseudoscience is that science is falsifiable but pseudoscience is unfalsifiable. Science is arranged so that any incorrect hypothesis will be exposed and corrected if it conflicts with experiment. Secure theories like special relativity have had hundreds of experimental confirmations over the years, and some of special relativity's predictions like time dilation test to extremely high accuracy every time they are tested. Pseudoscience is arranged so that any deviations from its predictions are readily explained within the internal logical precepts of the pseudoscience itself. Example: most modern astrologers reject total determinism. Rather than saying that the positions of the planets at birth determine personality and fate, they assign some weight to free will. ("The stars incline, the do not compel." See question 14 in the astrology FAQ.) In the event that a horoscope is perceived as inaccurate, and other excuses fail, acts of free will can always explain the inaccuracy. Another example: It is impossible to discount UFO sightings at the 100% level. It is clear that 90% are easily explained as obvious hoaxes or as sightings of other natural or man-made phenomena. With more work, about half of the remaining sightings can also be explained. This leaves about 5% of UFO sightings that are inconclusive. Even if the UFO community agreed with these statistics, which they don't, it is still possible for UFOs to exist because only one sighting has to be a real extraterrestrial.
