The Marauders were a group of four Gryffindors who attended Hogwarts school. While at school, they had a tight bond, created the marauders map, and pulled pranks - with the help of James' invisibility cloak. They, in the wizarding world, were infamous in their years after Hogwarts for their many fantastic pranks.
James Potter, our main character is a member of this rag-tag group. Lily Evans does not approve of their pranks. The Marauders are part of the reason she despised him for so long.
The four students that had a knack for rule-breaking and mischief making were:
Their nicknames, as seen on the Marauders Map, come from the animagus form of each respective member. Remus- however, is a werewolf. His nickname is a reference to the fact he turns into his wolf form under the full moon. His friends worked to become animagi so he wouldn't be alone in his transformations.
James - Prongs: Named for the antlers on his stag animagus.
Sirius - Padfoot: Named for the feet of his black dog animagus.
Peter - Wormtail: Named for the appearance of the tail on his rat animagus.