The Marauder's Map is a magical map of Hogwarts created by James Potter, Sirius Black, Remus Lupin and Peter Pettigrew while they were at Hogwarts, during which time they gained extensive knowledge about the school grounds, such as its various hidden passages, from their frequent night-time adventures together.
At first glance, the Map is simply a blank piece of parchment; but when the user points his wand at the Map and says, "I solemnly swear that I am up to no good!", the message "Messrs. Moony, Wormtail, Padfoot and Prongs, purveyors of aids to magical mischief-makers, are proud to present the Marauders Map", and a detailed layout of Hogwarts appears, including its occupants, secret passageways (and instructions on how to access them), and other mysteries, though several locations like the Room of Requirement and the Chamber of Secrets do not appear on the map, either because the Marauders did not have any knowledge of them, or—in the case of the former—they are Unplottable. Furthermore, Animagus disguises, Polyjuice Potion or Invisibility Cloaks cannot fool the map, as shown in Goblet of Fire when Barty Crouch, Jr. (disguised as Moody) breaks into Snape's office[HP4]. Saying, "Mischief managed!" returns the map to its original blank state.
In Prisoner of Azkaban, Fred and George Weasley, who have no further need for it, give the map to Harry so he can get to Hogsmeade through a hidden passageway. Fred and George stole the map from a drawer in Filch's office that contained dangerous confiscated objects; it is revealed by Lupin that Filch probably knew what it was but not how to work it. Snape later finds the map in Harry's possession and tries to force it to reveal its secrets, but the map merely insulted him with mocking phrases, much as the Marauders themselves would have done. Lupin, the current Defense Against the Dark Arts teacher, is called upon to investigate this "dark object", and confiscates it to keep Harry safe, though he returns it after resigning his post at Hogwarts. From then on, the map becomes one of Harry's tools in his ongoing adventures, and is mentioned only in passing when he consults it.
On the prop version of the map made for the films, the lines are made up of what at first glance are just random letters, but upon closer inspection are Latin words. The series makes no mention of Harry recovering the map from Moody's office, even though he continued to use it in later books; when asked about this discrepancy, Rowling answered that Harry had indeed sneaked into the office and recovered it in the days following the Third Task, and that she had forgotten to include this detail in on-page. When asked during an online question session, "What child did Harry give the Marauder's Map to, if any?" (after his school years), Rowling responded, "I've got a feeling he didn't give it to any of them, but that James (Harry's eldest son) sneaked it out of his father's desk one day."