A network segment can be somewhat different depending on the topology of the network. The easiest way to understand the concept is with bus networks because it is basically the same for all topologies.
A segment is simply a single length of network cable that extends to all nodes connected within that segment. This should not be interpreted as a single strand of cable because the cable is broken up between each computer at a connection point with a connector, which allows the computers to connect to the network cable. Even though the cable is broken, it is still an electrical single length of cable. Within the segment, all nodes are able to see the network traffic. This is why it is important that when planning, you take in consideration the number of nodes that you plan to connect to any one segment. If you plan to have 25 nodes on a segment, each computer will only receive 1/25th of the maximum available bandwidth.