There are many theories regarding what is variously referred to as aliens, extra-terrestrial, UFOs, flying saucers, etc. What is of interest is that there has been a point of convergence of sorts which has come about. This has occurred by people with very different theories somehow ending up with a theory of everything, as it were, pertaining to this particular topic.
In essence, there are three main categories into which all theories can be broken down:
1) Extra-terrestrial aliens
2) Demons
3) Human / government