A-B Repeat refers to that function or option of a hardware or software audio recorders/players that cycles or loops a section of a track.
Analog to Digital Converter (or simply put ADC, A/D or A to D) is that device which transforms analog audio signals into digital form / data. The reverse operation of this process is made by a Digital to Analog Converter (DAC). An Analog to Digital Converter is one of the most used devices in today music production industry since the computers are the core of today recording and production studios. Once converted into a digital form, digital audio signals can be processed in a lot of ways, the only limit being the software and the actual music production software solutions can reach the sky.