T568A and T568B refer to the pin/pair assignment of 8P8C connectors. Connectors are classified as 8P8C when the array of pins is distributed in an 8 Position 8 Contact format. The 8P8C male connector is usually referred to as RJ45 plug, and the female 8P8C connector is referred to as modular jack.
Balanced twisted pair cables in a structured cabling system consist of 4 pairs, hence 8 wires. Each wire terminates at the 8P8C connector in two ways: T568A (ISDN) and T568B (ALT). The difference between the two color codes is that pairs 2 and 3 are interchanged as shown in the image below.
Both are allowed under the ANSI/TIA wiring standard. T568A color code is preferred over T568B because it allows pairs 1 and 2 to be used intermittently. In all cases, a unifying color code scheme should be adopted for all modular plugs and connectors being executed under one project.