The document discusses number systems. It defines a number system as a system for writing and representing numbers using digits or symbols in a consistent manner. It allows for arithmetic operations and provides a unique representation for every number. The four most common number systems are decimal, binary, octal, and hexadecimal. Binary uses only two digits, 0 and 1, and is used to represent electrical signals in computers. Decimal uses base 10 with digits 0-9 in place values. [END SUMMARY]