With regards communication there are a few common terms used to describe the dynamics of the parties communicating. Full-duplex, describes a situation where both parties are capable of communicating simultaneously. Half-duplex, on the other hand requires that only one party can communicate at a time but communication are bi-directional. Simplex however is not bi-directional with it communication is only possible in one direction, therefore only the transmitter can communicate to a receiver. Examples of full duplex, half-duplex and simplex; are a telephone conversation where two parties are capable of talking at the same time, communication is bi-directions and simultaneous. That can be compared to a radio walkie-talkie where each party must take turns talking, the conversations is bi-directional but only one at a time person can talk at a time. Simplex is even more limited in so that only one party can communicate and in only one direction and it is a fixed order. An example of simplex could be a public address system.
Full-duplex is an important communication specification as it not only facilitates simultaneous two way conversation but effectively doubles the throughput capacity. Ethernet in data and VoIP environments are today always full-duplex but earlier versions of Ethernet started out as half-duplex and then it was something that needed consideration. In switched data networks Full-duplex ports have been the norm for years though they can be configured as half-duplex for backwards compatibility. However, such is the improved data throughput using full-duplex and the fact that there is no longer contention and collision issues with full-duplex Ethernet, half-duplex is rarely used except to support old legacy equipment.