What are some similarities and differences in the plot of the story "All Summer in a Day" and the movie All Summer in a Day?

If you are talking about the 1982 short film based on the story, there are significant differences. For one thing, Margot in the story is much more isolated. In the film she has a friend with whom she shares her treasures -- her secret cache of flowers and a butterfly from her life on Earth. In the story there is no such cache, nor is there any such friend. In the story, when the rain doesn't stop when they thought it would, the children lock Margot in the closet immediately; in the film, she is locked in the room after the rain stops -- a crucial difference. Of course the Venus in the story is very different from the "Venus" of the film -- in the story, the planet is a rain forest, not a flower-filled meadow. 

But the major change is the ending. In the story, the end comes when the children open the door to let Margot out. Bradbury doesn't say what will happen next. In the film, the end is much more positive and definitive: the children let Margot out; she is heartbroken. The children, one by one, recognizing what they have done, apologize to her by giving her the bouquets they picked while the sun was out. The film ends with William, the chief bully, desperate for foregiveness, giving Margot his flowers. Margot accepts them, and they walk off together into the rain, her arm around him.

The ending of the film totally changes (in my opinion) the tone of Bradbury's story. In the Bradbury world, children are brutal, vicious creatures who somehow are able to operate outside the bounds of adult control (see Wendy and Peter in his story "The Veldt" for another example). The open-endedness of the story actually supports this view. The point is that we readers have no idea what is going to come out of that closet at the end. In the film, however, Margot is made into an impossibly tolerant person, who transforms the children through her ability to suffer and forgive. Based on the comments about the film on IMDB, many people of a certain age remember the film version as "life changing" -- but I much prefer deciding for myself what Margot is going to do next!