ALOHA History Section
An Aloha station could send whenever it liked, and then waited for an acknowledgement. If an acknowledgement wasn't received within a short amount of time, the station assumed that another station had also transmitted simultaneously, causing a collision in which the combined transmissions were garbled so that the receiving station did not hear them and did not return an acknowledgement. Upon detecting a collision, both transmitting stations would choose a random back-off time and then retransmit their packets with a good probability of success. However as traffic increased on the Aloha channel the collision rate would rapidly increase as well.
Follow this link to see a more detailed animation of the ALOHANet.