The Marauders: Everything the Books Tell Us
Who they were
The four Marauders, James Potter, Sirius Black, Remus Lupin and Peter Pettigrew, were best friends at Hogwarts in the 1970s. They created the Marauder's Map, one of the most useful magical objects in the series, and they became illegal Animagi so they could keep Lupin company during his monthly transformations as a werewolf.
The books give us their story in fragments, mostly through other people's memories. Lupin and Sirius fill in some gaps, Snape's memories add a much less flattering angle, and Dumbledore provides context. Piecing their history together is one of the great fan pastimes.
What went wrong
The friendship collapsed because of Voldemort's first war. Someone in the Potters' inner circle was passing information to the enemy, and Sirius suspected Lupin, which is why the Potters switched their Secret Keeper from Sirius to Peter at the last moment.
Peter betrayed them. The Potters died. Sirius was framed and spent twelve years in Azkaban. Lupin spent those same twelve years believing his best friend was a murderer. The tragedy isn't just that Peter betrayed them; it's that suspicion had already broken the group before the betrayal happened.
This is the backstory that makes the third book work. When Sirius escapes Azkaban, the reader thinks he's the villain. The reveal that Peter is alive and that Sirius is innocent is the best twist in the series.