There are two types of representations used in digital systems, the positive logic and the negative logic representations.
In positive logic representation Bit 1 represents Logic high and Bit 0 represent a Logic low as shown in fig 2 a and b. High is represented by +5 Volts and low is represented by -5 Volts or 0 Volts.
In Negative logic representation Bit 1 represents logic low and Bit 0 represents logic high as shown in Fig 3 a and b. In terms of voltage level, bit 1 can be represented as +5V and bit 0 can be represented as 0 V or -5 Volts