A Broadcast is a message intended to reach an infinite number of users.
Broadcasts are encrypted with a hash of the information encoded in the sender's Bitmessage address. Anyone who knows the sender's address will be able to decrypt the broadcast. This prevents unencrypted broadcast data from flowing through the network connections and being stored on the disks of uninterested Bitmessage users. pubkey objects, which are distributed throughout the network, are encrypted in the same manner.
Broadcasts are not acknowledged with an ACK Message. They are lost after about 2 days.
Broadcasts are not resent. Resending has to be done manually by the user if he wishes to do so but this is considered to be a completely different message from the network point of view.