Pure ALOHA and slotted ALOHA are the two different variations of of ALOHA.
Pure ALOHA mean transmit whenever you have data. Slotted ALOHA means wait for your time to transmit i.e. time is divided into discrete intervals and each interval corresponds to a frame of data. It does not allow a system to transmit any time. Instead the system has to wait for the beginning if the next slot.
Pure ALOHA is featured with the feedback property that enables it to listen to the channel and finds out whether the frame was destroyed.