Greater love has no one than this: to lay down one's life for one's friends. (John 15:13 TNIV).
Of course the greatest example of this kind of love is Jesus himself:
For even the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many (Mark 10:45).
This is how we know what love is: Jesus Christ laid down his life for us. And we ought to lay down our lives for one another (1 John 3:16).