Of zero's and one's. Binary notation starts with the 2 being the number base. A base ten system goes from 0 to 9, while a binary goes from 0 to 1 without repeating a digit. In base ten, the number 123 is represented as one 100, two 10's and three 1's. Each being a multiple of ten.
Binary notation is perfect for describing positive numbers and zero.
But when we want to allow for negative numbers, an additional mechanism is needed to indicate the sign of the number. The easiest way to do this is to use the leftmost bit to indicate the sign of a number. A problem is introduced when doing binary subtraction however. The format of each is the same except for the length, thus only the range is different for each. Two's complement is used for negative numbers.