This list was created by the AFI (American Film Institute) in 1998. 1998 was the 100 year anniversary of the first American film. So in honor of this great occasion the AFI picked the 100 greatest films of the first 100 years of movies. I have this list hanging on my wall. I try to rent something off it that I haven't seen every now and then. I've probably seen half the movies on it.
This list caused much controversy at the time it was released. As some people's favorite movies were not included or something they hated was.
Really this is just one group's opinion of what the greatest movies were. You are still free to have your own opinion nobody is stopping you. "Greatest" is such a subjective word.