A.  T568A and T568B are the two color codes used for wiring eight-position RJ45 modular plugs.  Both are allowed under the ANSI/TIA/EIA wiring standards.  The only difference between the two color codes is that the orange and green pairs are interchanged.   T568A wiring pattern is recognized as the preferred wiring pattern for this standard because it provides backward compatibility to both one pair and two pair USOC wiring schemes.

The T568B standard matches the older ATA&T 258A color code and is/was(?) the most widely used wiring scheme.  It is also permitted by the ANSI/TIA/EIA standard, but it provides only a single pair backward compatibility to the USOC wiring scheme.  The U.S. Government requires the use of the preferred T568A standard for wiring done under federal contracts.  The following diagrams look at the jacks from the front.  The wiring at the rear of the jack varies by manufacturer and may not be in the same sequence as the front.  However, compliance with the color codes is maintained by routing the connections at the back to the proper sequence at the front of the jack.  That is usually done by a small printed-circuit board in the jack assembly.  CAT 5e jacks may have a twist inside the jack to reduce crosstalk.