Alien is usually used to distinguish a character or starship that is NOT from one of the primary or known governments. In particular, Alien serves as a category placeholder that is distinct from the common ones of Federation, Klingon, Romulan, Borg, and Cardassian... Repeating, being Alien is more about the Category:Governments...

However, it might also be used as a general adjective. Regardless of how you ended up at this page, this will not hold a list of all the different aliens in the game - so you will probably want to go to:

note: Ferengi and Vulcan are both currently in-between being common and being alien (see: Weapons...)