Who snitched on Lily Potter?

Answer 1

Severus Snape was the person who overheard part of the prophecy and informed Lord Voldemort of it. Once he reaslised Lily, the woman he always loved, was in danger. He told Albus Dumbledore so he could protect the Potter's.

They went into hiding and used the Fidelius Charm which requires trusting someone with a secret which only they can tell. Orginally, they planned on using Sirius Black for this task but at the last minute decided that Sirius was the obvious choice and he'd likely be captured and tortured so switched to Peter Pettigrew without telling anybody.

Unknown to them, Peter Pettigrew was working for Lord Voldemort and he betrayed them leaving Sirius to take the blame.