the ones I saw were
1- Global Warming ( the amount of CO2 is increasing from industry ect causing the earth to warm and melt ice ect, which would cause major flooding in major area of the world
2- Nuclear war- they said something about earth has enough nuclear weapons to kill 8 billion people and there are 6 billion people on earth, and that it could only take an accident for a weapon to get launched and destroy humanity
3- Disease or sickness, pandemic such as the Bird Flu and virsues that could get into the wrong hands and be replicated and kill mases of people
4- huge Asteroid- you get the gist of that, you can't blow it up like in the movies, it would cause more damage. A theory is that if an asteroid ever does head are way they might be able to use machinery to knock it off its course, but there isnt even a blueprint for that yet.
there are 3 or 4 others I missed (black holes-they can actually roam, volcanoes, artifical intelligance one day will be smarter than humnans?) if you go to abcnews.com you then to 20/20 you can read the Hawking interview, that dude is a genius and he explains it best, our greatest threat is ourselves.