Number systems provide the basis for all operations in information processing systems. In a number system the information is divided into a group of symbols; for example, 26 English letters, 10 decimal digits etc. In conventional arithmetic, a number system based upon ten units (0 to 9) is used. However, arithmetic and logic circuits used in computers and other digital systems operate with only 0's and 1's because it is very difficult to design circuits that require ten distinct states. The number system with the basic symbols 0 and 1 is called binary. ie. A binary system uses just two discrete values. The binary digit (either 0 or 1) is called a bit.
A group of bits which is used to represent the discrete elements of information is a symbol. The mapping of symbols to a binary value is known a binary code. This mapping must be unique. For example, the decimal digits 0 through 9 are represented in a digital system with a code of four bits. Thus a digital system is a system that manipulates discrete elements of information that is represented internally in binary form.