On 31 October in 1981, Voldemort entered their home. James told Lily to take Harry and leave while he held him off. James, however, was killed before Lily and Harry could escape the house. Trapping mother and son in Harry's nursery, Voldemort, honouring Snape's request to spare her life, offered Lily a chance to step aside. She steadfastly refused to stop shielding her son, and Voldemort killed her. Because Lily's death was a willing sacrifice, not a mere casualty of war like James's, Harry was marked with the protection of her love, and Voldemort's Killing Curse rebounded off the one-year old, destroying the Dark Lord's body and temporarily defeating him. If not for Snape's love for Lily, which led to him to beg Voldemort for Lily's life, her death would not have been a willing sacrifice, and there would have been no Boy Who Lived.

Taken from http://harrypotter.wikia.com/wiki/Lily_Evans

I've never fully appreciated the significance of Snape. Can't stop thinking about him since DH and now the track 'In the end' is stuck in my head...