Simplex and duplex transmission

This is a type of transmission in which the data can only flow in one direction only.

This is a type of transmission in which data can flow in either direction, but not at the same time.

Each unit at the ends of the transmission link can act as either a receiver or as a transmitter, but only one at a time.

This is a type of transmission in which data can flow in both directions at the same time.

Each unit at the ends of the transmission link can act as both a receiver and a transmitter at the same time.

The terms simplex, half duplex, and full duplex can apply to any kind of communications channel.

So for example :-

A paging system is a simplex communications channel, as the paging receiver cannot be used to send back a signal.
A commercial two way radio channel is an example of a half duplex system - pressing the push-to-talk button turns the radio into a transmitter, releasing the push-to-talk button turns the radio into a receiver.
the normal telephone system is a full duplex system, as both parties can talk and listen at the same time.