A network segment is a logical group of computers that share a network resource. This can be accomplished with a router, VLAN, switch segmentation, etc. Unfortunately, with a hub, everyone sees every packet which is why they have pretty much died in the market. Switching is a much better technology. You can segment a network either logically (through VLANs or mapping) or physically (connecting switches back to a core).

If you can picture all connections coming into a hub, the packet travels through every port. With a switch, it can read the MAC info, or in the case of a Layer 3 switch, the IP address info, and it will skip over any ports that do not need the packet and switch it over to the port that does through the switching fabric.