"Your devotion is nothing more than cowardice. You would not be here if you had anywhere else to go."
-- Lord Voldemort (GF1)
Wormtail was the servant of Voldemort who found him in Albania and helped bring about the Dark Lord’s rebirth and return to power.
While at Hogwarts, Peter Pettigrew first used the nickname "Wormtail" when, with the help of Sirius Black and James Potter, he learned to become an Animagus (a rat) (PA17, PA18). However, after leaving school, he became allied to Lord Voldemort and passed inside information from the Order of the Phoenix to him. He revealed James and Lily Potter's location to Lord Voldemort, which led to their deaths and Voldemort's disappearance. While presumed dead himself and in hiding, Wormtail was the pet of the Weasley family, who called him "Scabbers", feeling that he would be more likely to hear word of Voldemort while living with a wizarding family.
In June of 1994, Peter Pettigrew finally had his reckoning with Sirius, Remus Lupin, Harry, Ron, Hermione, and Severus Snape in the Shrieking Shack, and Scabbers' true identity was revealed. Sirius and Lupin would have killed him, but for Harry's intervention. Harry wanted to clear his godfather's name and insisted that Peter be held for trial instead. Wormtail now owed Harry a "life debt" (PA18).
Unfortunately for Sirius, Wormtail was able to escape when Lupin's transformation into a werewolf distracted his captors. The first thing he did was to go searching for his dark master, Voldemort. Amazingly, he found him, existing bodiless with a snake named Nagini -- in Albania.
Even with a new servant, the next step wasn't clear until Wormtail ran into Bertha Jorkins, a Ministry witch who was on holiday in Albania. Although she recognized him, Wormtail was able to lure her into the forest where she fell into the clutches of the Dark Lord. Under torture, Jorkins gave them information that helped Voldemort make plans for an attack on Harry Potter.
Around this time Pettigrew seems to have dropped his birth name completely and become Wormtail permanently. On Voldemort's instructions, Wormtail created the potions his master needed to gain a temporary body. Wormtail milked the venom from Nagini that was used as part of the potion. He carried Voldemort's weak, small body on the long journey back to Britain, where they took up temporary residence in the Riddle House outside of Little Hangleton (GF1).
When Harry Potter was captured, it was Wormtail who killed Cedric Diggory, Wormtail who bound Harry, Wormtail who set up the cauldron over the fire, and Wormtail who cut Harry's arm to draw for blood for the potion. Wormtail then cut off his own right hand to add to the potion. From that horrible mixture, the Dark Lord was reborn in his old body. He cast a spell which created a new shining and powerful hand for Pettigrew (GF33). Voldemort used the Dark Mark to call the rest of his wayward Death Eaters to him.
From this point on Wormtail was something of a lowly servant to Snape and the other Death Eaters (HBP2, DH1). He met Harry again in Malfoy Manor and tried immediately to kill him, but when Harry reminded Wormtail of his life debt, Wormtail hesitated – and that was enough for the hand created by Voldemort to sense the failing, turn back on its owner and betray the betrayer. Wormtail was choked to death (DH23).
Wormtail was an Animagus who could turn into a rat.
Other canon notes and references
"How do you think of all the names, like Moony, Wormtail, Padfoot and Prongs?
Those names all came out of the creatures that they turned into. I had a lot of fun with those. Wormtail was the most difficult one. My sister loathes rats and her problem with them is their tails, so that is what gave me the idea."
-- J. K. Rowling (EBF)