A UFO is an unidentified flying object. In reality, they're fake, if you're thinking of aliens. Wouldn't you think that with our technology today and our more accessible photography capabilities you would see more pictures (and more clearer) of ufos but you don't.
The most compelling piece of UFO was a shot of spacecraft identified in Israel in 1988. The craft banked on either side and people taking shots of it were able to view it at different angles. Visit http://www.ufodigest.com/bestof.html for more on UFO sightings.
Now, I'm not saying that I've personally seen any Alien spacecraft, but thousands of people (including the Governor of Arizona) saw the 'Phoenix Lights' back in March of 1997 - and no one can explain that occurrence.