I think you maybe talking about an Ethernet splitter. For 100Mb/s commonly called fast Ethernet you can use the 4 pair cable for two Ethernet runs or part of the cable for Network and part for phone lines.
Fast Ethernet uses pairs 1,4 and 3,6. The other pairs are actually unused.
An example:
You have one CAT 5 cable run from your switch to desktop. You add another computer to that location. You would just run anther CAT 5 cable for the second computer. A quick solution is to use a splitter at each end. Now your using that one CAT 5 cable for two computers.
This is not the case for Gigabit Ethernet commonly used today. It uses all four of the pairs. So this is never really seen or used anymore.
Hope this helps.