The map is basically an ordinary folded up piece of parchment, but as soon as you say "I solemnly swear as I'm up to no good", the description and the map layout of Hogwarts will magically appear. People are represented by their names and their footprints. After you finished with it, you say "Mischief managed", and the map will return to its ordinary parchment.
Harry attempted to sneak into Hogsmeade with his Invisibility cloak, Fred and George caught him and show him the Marauder's map that they stole from Filch's office. Harry uses the map to get around Hogsmeade, and finds out (under his invisibility cloak) that Sirius Black betrayed Harry's parents to Voldemort.
Much later, Harry noticed Peter Pettigrew's name on the map, and searches around the corridors for him. Unfortunately, he was caught by a passing Snape, who forces him to reveal his secrets. After Harry uses the map to insult Snape, Lupin arrives, and Snape promptly confiscates the parchment from Harry, handing it over to Lupin, claiming it is a "Zonkos" product. Lupin takes Harry to his classroom to not just to lecture him about the responsibility of not handing in the map, but reveals that his father was a troublemaker in his childhood from Lupin's view. He asks Harry to return to his dormitory, and before Harry leaves, he informs Lupin that the map isn't working properly, and that he saw Pettigrew's name on the map. After Harry left, Lupin quietly looks at the map and at some time, eventually realizes that Harry may be right of Pettigrew's name.
Lupin still kept the map in his possession for the remainder of the film, until as Harry visits him in his office, Lupin, with no regret, returns the map back to him, before resigning from his job.