A nibble or nybble is a digital unit of measurement that refers to four binary digits or half a byte (4 bits). Since there are only 16 possible values for a nibble, it can be expediently represented by a single hexadecimal digit.
A bit is the smallest digital unit of measurement, and it only has two possible values, "off" and "on." "Off" is typically represented by 0, while "on" is represented by 1. A byte refers to a standard group of 8 bits. The largest number that can be represented as a byte is 11111111 and the least is 0000000. A group of bytes is called a "word."