Good definition of science fiction from the Wikipedia: "Science fiction is a genre of fiction dealing with the impact of imagined innovations in science or technology, often in a futuristic setting. It differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation). Exploring the consequences of such differences is the traditional purpose of science fiction, making it a "literature of ideas". Science fiction is largely based on writing rationally about alternative possibilities. The settings for science fiction are often contrary to known reality, but the majority of science fiction relies on a considerable degree of suspension of disbelief provided by potential scientific explanations to various fictional elements."
Some famous Sci-Fi movies include Metropolis (1927), 2001: a Space Odyssey (1968), Star Wars: a New Hope (1977 and its sequels); Close Encounters of the Third Kind (1977, 1980), E. T., (1982), and many more. Most of those listed by Syler are not truly science fiction.
Attached is also Wikipedia's article on sci-fi films; it has a very good list.