Segments
In networking, the term
"segment" has several meanings: First, in a narrow sense, it
can refer to a trunk (main line) of cabling, which connects devices to
a concentration device (hubs, MAUs, or switches). Second,
"segment" can also refer to a logical grouping of devices,
which communicate within a given subnet separated
("segmented") by bridges, switches, or routers. The term
"segment" is sometimes synonymous with a collision and/or
broadcast domain 
Backbones
The term "backbone" also has
several meanings within networking. First, a backbone is most often the main
cable (or trunk) to which all nodes and devices connect. Second,
backbones are
the foundations of both LANs and WANs where servers, routers, and
concentrating devices (such as switches and hubs) are connected by a
high bandwidth connection. Because of its desirable electrical
characteristics, such as immunity to noise and grounding problems,
optical fiber is now more commonly chosen for backbone cabling over
coaxial cable and UTP. 
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