Data is transmitted in one direction only
A terminal can only send and not recieve data or receive and not send data at one time. Today, this mode of transmission is not popular because many modern communications require two-way exchange of data. This is faster and more effective.
An example is a radio station, you cannot contact the radio station through their transmitter because it only transmits, it doesn't know how to receive.
Data can be transmitted in both directions but only one direction at a time
One terminal receives/sends while the other sends/receives.
An example is a walkie-talkie, only one of the walkies can talk to the other, they both cannot talk to each other at the same time.
Data can be transmitted/received in both directions at the same time
It is the fastest directional mode of communication.
An example is the telephone as both people who are connected are able to speak over each other showing the phone receiving the other persons voice as well as sending the phone owners voice.