Friends are a feature in Godville where you can invite new gods by typing their email address, or existing gods by typing their username. Its only possible to do this if you have the item Invitation to Godville or bey a gift of free invites by the adimns.
Once you have a friend, you can send messages to each other, apart from forums and voices during duels, this is the only way for two gods to communicate. Once an invite is sent, the god is added to your friend list, confirmation is not required.
There are also a number of random events that may happen once you have a friend. Your hero might meet the other god's hero, get together and smoke or drink, healing some health. Your hero might run into the friend and borrow some health potions, healing some health. During battle with a monster, the friend might finish killing that monster for you.