Anyone who has read HG Well's timeless SciFi Classic, The War of the Worlds and seen this movie will be able to tell you there are a very large number of differences.
This movie has taken the story and warped almost beyond recognition, changing the Martians to some unknown alien race, changing the setting from 1890s England to 2000s America, and Changing the main character from a Nameless yet sophisticated English Journalist to a Short, American Crane Operator, played by Tom Cruise who sets out to stay alive and keep his kids alive. In the novel, the main weapon of the martians is, of course, the deadly heat ray, replaced in this film with some sort of Dessication Ray, fired from hair dryers attached to the alien's tripods.
This movie has some brilliant effects, but it is let down by the Americanization of a timeless story. I would appreciate it more as a movie if it were not named the War of the Worlds because it bears little resemblance to the book, and would probably receive a lot less criticism from loyal fans of the book and other, more faithful adaptations.