Description
Simplex means the information is sent one way only, usually from the sender A to receiver B and cannot be switched back. Duplex means the information can be send both way, from A to B or B to A, or both at the same time.The word simplex might be derived from 'Sim' which means 'Simple' and 'plex' which means 'Part of' or 'Structure'. The word duplex is derived from 'Du' which means 'Duo' and 'plex' which means 'Part of' or 'Structure'.
Simplex
Simplex sends the information one way only and the path is fixed. Examples are TV and radio broadcasting, where the receiver only receive but not send information.
Full Duplex
In full duplex, both parties can communicate simultaneously. Telephone is a type of full duplex communication tool, as both parties can be both sender and receiver (To speak and heard at the same time).
Half Duplex
Half duplex the other hand allows information to flow both way, but not simultaneously, which is, one party is the sender while another must be the receiver. Walkie-talkie is a half duplex communication tool, as a talker will push a button called 'Push-to-talk' to turn off the receiver but turn on the transmitter.
This article is referred from Wikipedia's Simplex communication and Duplex (telecommunications) article.