The climax of the story is the turning point of the story; the moment when the ultimate suspense reaches its peak. The climax is the problem of the story.
Many believe the climax is the most intense or interesting part of the movie. This is a common mistake as the climax is when the theme is proven, which commonly is the most interesting or memorable scene. After the climax, the story cannot be proven anymore, or there is no more story to tell other than the conclusion. [Example Lord of The Rings - when the hobbit finally destroys the ring is the climax. The theme is, "Even the smallest person in the world can make the biggest difference."]
But it happens sometimes that story seems to end however another turning point is awaiting (as in Van Helsing, The Dark knight...) so one should never conclude that there is only one climax in a movie.