Unfortunately, watching the trailer above all the way through is no good—you've got to head to the Youtube page itself so you can make sure you turn annotations on. That way, you can experience this "interactive" XCOM: Enemy Unknown trailer the right way.
It's essentially a choose-your-own-adventure of trailers; click the left choice at the beginning and the video continues, but click the choice on the right and it jumps ahead. It's actually quite clever.
Overall, you'll get a sense of just a fraction of the decisions you'll be making in every mission of XCOM: Enemy Unknown, the re-imagining of the classic 90s strategy series. Of course, if you want an even better idea of how the alien invasion story will play out, you can also try out the demo on Steam.