Most people know the alien and predator extra-terrestrials(?) from the Sigorney Weaver and Arnold Schwarzenegger films of the 1980s, but a few years ago some Hollywood bright spark thought it'd be a good idea to pit one against the other in a battle to the death to see "which is best" (and hopefully get a new franchise out it). In case you've forgotten I think the same studio made a movie that saw Freddy Kruger fight Jason from the Friday the 13th movies at about the same time. It, like this film, was dire.
This film is the second in the combined aliens franchise and sees the Predator accidentally crash land into Colorado when the Alien gets loose on his (or her) ship. The results, perhaps unsurprisingly, aren't pretty.
The only problem is that there's no real story after the Predator accidentally crash lands on Earth and you're only likely to understand that if you're aware of te previous films in the series.
To make matters worse, the script is really dire (lots of wooden dialog and women screaming with not a lot on) and the actors don't get us to care about the people in the story (though this might be in part due to the story and the wooden words they have to say). So all in all a really bad movie.
Incidentally, it took me till mid-December to see the worst film of 2007 (The Mist). The very first film I saw in 2008 is worse. Trust me, when you're saying that a film is worse than "The Mist," you're really saying something.