The after party of a birthday including the birthday participant and the birthday participants friends. Usually an invitation to this kind of party means that you are a close friend/lover of the opposite sex and are most likely in for a long night. Many a time have boys received invitations to after birthday parties and have come out different men, due to the wild/craziness of the sex.
Papa Johns: Yo steve, how was the birthday party?
Steve: The party was insane, I was invited to the birthday after party and now I am a changed man.