In Simplex Transmission, data flows only in one direction - from the sending device to the receiving device. Simplex Transmission is used only when the sending device does not require a response from the receiving device.
Example: A microphone to a loudspeaker
Half-duplex communication allows two-way conversations, one-way at a time, such that one person cannot interrupt the other.
Example: A walkie-talkie
Full-duplex describes bi-directional communications all the time. Regular communications between individuals conversing face to face is full-duplex. In other words, you can talk and listen simultaneously. Full-duplex communication allows simultaneous two-way conversations where one person can interrupt the other.
Example: Two people having a conversation.