Bob,

As a retired USN radioman I'd like to take a stab at this. If the few
remaining grey matter cells will
cooperate. I didn't read the Wiki article, I'll do that next to see if it's
correct. GGGGGGGG

Simlex is a one way only circuit. Like a fleet broadcast. Or a TV or Am/FM
radio station. It uses a single freq.
One side transmits and if all goes well the other side is able to copy
what's being sent. It's not always necessary
to have two way comms. Cable TV used to be basically a simplex ckt but know
there's all sorts of Buck Rogers
stuff out like interactive cable.

Duplex is silmultaneous comms both ways. It requires two separate freqs, one
for send the other for receive.
A ship shore RTTY circuit on HF is an example. Each side can be "talking"
to the other side using a TTY machine.
(Okay so TTY machines went out with the buggy whip. It's still a valid
concept.)

Half Duplex is as the name suggests nothing but back and forth comms on a
single freq but NOT at the same time.
Two way radios are a good example. One side talks and other listens and then
the other side talks and the first side listens.

Have you ever tried to talk on a phone while the other person is talking?
Sure, but it doesn't work because you
both end up talking on top of each other. So yes it's 'possible' for both
sides to talk at the same time but this is not
a very good solution if the idea is to communicate. When you get into AM
modulated voice when you combine two
voice freqs the two freqs actually add and at the same time subtract from
each other. I realize that may not make
sense but I don't want to get too technical. Am sure Wiki can and has done a
much better job that I can.

Before everyone jumps all over me I'm talking about very basic ideas and
not any of the more modern modes
like cell fones and digital and trunking radio systems and satellite etc etc
etc. As someone once said "the times they
are a changin'" but no matter how marvelous things get the basics will
always remain the same.

Regards,

SparkS