To inform. The review needs to tell people who is in the film, who it is by and where or when readers can see it.
To describe. The review should describe the story, characters and some of the action - without spoiling the plot or giving too much away!
To analyse. A good review gives an opinion on whether the film is good or not and why.
To advise. Finally, the review should tell the reader whether or not to go and see the film