Abhishek,
Technically, RJ-45 refers to just the modular connector–just like RJ-11 refers to the smaller “phone” type connector. The ethernet cable itself would be referred to as CAT-3, CAT-5, or CAT-6 (most modern cable). The length limit of CAT-5 / CAT-6 cable is 100 meters. For longer than that, you would have to use fiber. As far as the length of fiber, that depends on what kind of speed you are trying to push over it. The higher the speed, the shorter the distance. For example, you can push 100 Mbit/s over 2 kilometers, while 10 Gb/s drops the distance all the way down to about 500 meters.
Ethernet is a layer 2 (data-link) standard, so it is independent of layer 1 (physical). Therefore, you can run Ethernet on Fiber or CAT-5 / CAT-6.
POS = Packet over SONET. These are popular (and very expensive) long haul circuits that are point to point in a WAN environment.