Copper Ethernet cabling is typically constructed with Category-5 (a.k.a. Cat-5), Cat 5e or Cat 6, each of which are rated for higher bandwidth capability than the previous. They are made up of four pairs of copper wires, each insulated then tightly and precisely twisted together (leading to the term Unshielded Twisted Pair or UTP) to make the impedance characteristics (and hence the behaviour at very high frequencies) of the cable very precisely known.
The pin positions of the pairs in the T568A and T568B connectors (RJ-45) are shown below, as well as 'crossover' cable connections. I found it useful to remember that pairs 1 and 4 never change position, no matter what cable type, and that pairs 2 and 3 swap position in a cross-over. Regardless of the connector used at either end, this is easy to remember, and leads to the well-known connection pattern, of a crossover cable, of 1-3, 2-6, 3-1, 6-2.
Straight-through is self-explanatory,where the same connector type is used on both ends (either T568A or T568B), and the pin positions of pairs 1-4 do not change.
In terms of pair numbering within the connector, I try to remember that pairs are numbered outward from the centre toward the 'top' (i.e. the lower numbered pins) for T568A i.e. 1,2,3,3 and from the centre to the 'bottom' 1,2,4,4.
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