Who are the Marauders??
They are Moony, Padfoot, Wormtail and Prongs - The makers of the Marauders Map. This map is used to see who is moving about the school and it also shows all the secret passages. Three Marauders - Padfoot, Wormtail and Prongs made themselves animagi so that they could keep their dear friend Moony company when he transformed into his Werewolf form.
The picture below (made by me ^_^) is the Maurauder's Map at the point when Snape tries to find out how to work the map in "Harry Potter and the Prisoner of Azkaban". It may take a while to fully load so please be patient.
moony - remus lupin
We first meet Professor Lupin in book 3 on board the Hogwarts Express when he wards off a dementor. Throughout the school year Lupin helps Harry to stop the dementors from making him pass out. About once every month, Lupin is taken ill and given a potion from Snape to help him get better. It isn't until near the end of the book that we find out that Lupin is actually a werewolf!!
padfoot - sirius black
Sirius Black is first mentioned in the first chapter of book 1 but he doesn't become a main character until book 3 when he escapes from Azkaban. Black is supposedly the man who got Lily and James Potter killed in the first place because he was their secret keeper. He told Lord Voldemort of their whereabouts and that is where you-know-who met his downfall when he failed to kill the Potter's young son Harry. Everyone lives in fear of him when he escapes but at the end of the book we find out that he has been innocent all this time!! After he told the Potters to reconsider their secret-keeper they chose their other close friend Peter Pettigrew (Wormtail) and he was the one who was a traitor.
wormtail - peter pettigrew
We don't meet Wormtail until the end of the 3rd book. When we do it's hard not to hate him as we find out that he was the one who got James and Lily Potter killed and not Black. Wormtail has the ability to change into a rat and he lived as the Weasley family's pet for 12 years whilst in hiding as this creature.
prongs - james potter
James Potter was Harry's father. He was married to Lily Potter but they were tragically killed by Lord Voldemort when Harry was a baby. Until the end of the 3rd book it was thought that Sirius Black as their secret-keeper betrayed them and told Voldemort of their whereabouts, but it was in fact Peter Pettigrew who wronged them.
Sirius Black is an Animagus and has the ability to transform himself into a dog.