Simplex, half duplex and full duplex. These are names given to the manner in which the data is carried across the communication channel. In the early days of data communication all the bandwidth was limited and it was sensible to use a technique known as half duplex, where data it transferred in only one direction at a time. The whole channel was used up by moving the data.
A similar situation exists when we make use of a 2 way radio. Both handsets have the ability to transmit but they cannot receive when they are transmitting. This is why special protocols had to be developed for using a 2 way radio. You will hear people say "over" when they are done talking, this is an indication that someone else can have a turn to talk. If you have ever used a 2 way radio, you will know that the handset is silent when you press the button to allow you to talk. This is known as half duplex communication.
It was later possible to improve the channels of data communication and allow data to move in both directions on the communication medium at the same time. This is known as full duplex. When you are on phone with someone, you have full duplex communication because you can both talk at the same time and hear each other talking. You may not be listening but you can at least hear. All modern data communication media and supporting protocols are capable to full duplex.
Simplex communication occurs when some data is broadcast but no return channel is provided. This is how we recieve our Radio and TV data. You can receive but you cannot send.