What would happen if an asteroid hit the earth?

I had seen an article on Yahoo saying that Obama really wanted NASA to concentrate on meeting a certain asteroid that scientists believe may have a chance of colliding with the earth in 2025. I was wondering if ALL of life on earth would cease to exist or just a portion? Is it even possible that they can throw it out of our orbit? I have seen the movie "Deep Impact" but that was a comet they were talking about!