Repeaters are useful for propagating a network signal, a signal comes in on an input port is often output to many ports. However since they add some delay to the transmittal of packets they reduce the maximum size a segment can be. However, repeaters can simplify the design of a network. Devices such as bridges and routers, which regenerate the signal, allow you to build larger networks. Bridges and routers work at different levels of the network.
Bridges work at the ethernet frame level while routers work at the protocol level.
In both cases, the bridge or router, has the property of filtering traffic and only transmitting the signal onto networks where it makes sense. Thus, in each case they have the effect of reducing unnecessary traffic. The IEEE 802.2 spec defines the general properties of ethernet. Subsuquent standards define how each media type will operate. The overwhelming majority of connections made today use twisted-pair wiring.