The ABC was a digital computer, so-called because it processed data in discrete, digital units (the digits 1 and 0).  It also used the binary (base 2)  number system in computation, the results being converted to and from decimal (base 10) for the purposes of human consumption.  Being binary, the data could easily be represented electronically since switches natural have two states—on and off—which lend themselves to representing the numbers or values 1 and 0.  The ABC used vacuum tubes, punched cards and a memory device that looked like a drum (shown in the pictures on the slide being held by Atanasoff and as it was located in the working machine).