Ethernets are very democratic in their access control mechanism, which determines who gets to use the coax wire and when. Instead of the dictatorial authoritarianism of a bus arbiter, all computers that are attached to an Ethernet can just go ahead and use it. Of course, this could result in chaos if more than one computer, through the electronics of its attached transceiver of course, tries to send bits over the wire. So a technique called CSMA/CD is used by all attached computers.

Here is what CSMA/CD stands for:

CS -- Carrier Sense     The attached computers sense (test)
                        the wire before they begin transmitting
                        to see if it is already busy.
                        If so, they wait.

MA -- Multiple Access   All computers have equal access to the
                        wire.  There is no hierarchy or arbiter.

CD -- Collision Detect  If two computers send at the same time
                        they will detect a collision of their
                        respective packets.