Half-duplex is a type of two-way transmission, where you can send or receive in only one direction at a time. Sending and receiving data simultaneously is not possible in half-duplex transmission. Ethernet Network operates in half-duplex, when a shared medium is used for communication. Example: When network devices are connected using bus topology or when hubs are used to connect devices in a network.
Ethernet has to fall back to CSMA/CD protocol, to operate in half-duplex. Half-duplex way of data transmission in Ethernet is not efficient when compared with full-duplex. Available bandwidth speed is far less when Ethernet operates in half-duplex mode. Ethernet networks operated in half-duplex mode during the early days of Ethernet.
Full-duplex is another type of two-way transmission in Ethernet networks. Devices connected in full-duplex Ethernet mode can send or receive data simultaneously. To operate in Full-duplex mode, Ethernet requires at-least two pairs of wires. One pair is used to send data and other pair is used to receive data, simultaneously. These days, FastEthernet, Gigabit Ethernet and 10 Gigabit Ethernet operates in full-duplex mode.