The great thing about this film is that, unlike many horror films, it doesn't matter how often you watch it. The shocks may become slightly more predictable (though the "head out the porthole" scene still ranks in my all-time top 10), but the fabulous characters and tension are in no way diminished. I won't bother with a storyline, since you undoubtedly know it, but this is an interesting deviation into what's more a two-act structure than the usual three. The first half is the political machinations around the earlier attacks, with the mayor arguing for the beaches to stay open, while the police chief (Scheider) and sharkologist (Dreyfuss) want them closed.
The second half is Moby Dick: the two men teaming up with professional hunter Shaw to take on the monster, trading battle scars, stories and expertise. Moments to treasure include Scheider's face after encountering the monster shark, Dreyfuss' crushing of a plastic cup, and Shaw's USS Indianapolis tale, plus John William's score. Never have two notes (E and F, to be exact) been so permanently claimed - more than 25 years later, they're enough for everyone to know exactly what you mean. The only flaw is the shark itself, which never looks even mildly realistic, and is a good argument for the "less is more" school of horror. That aside, it remains a true classic, with its status in no way diminished by the steadily more appalling sequels which followed.