Binary
Binary number is a number expressed in the base 2 numeral system. Binary number's digits have 2 symbols: zero (0) and one (1). Each digit of a binary number counts a power of 2.
Decimal
Decimal number is a number expressed in the base 10 numeral system. Decimal number's digits have 10 symbols: 0,1,2,3,4,5,6,7,8,9. Each digit of a decimal number counts a power of 10.
Octal
Octal number is a number expressed in the base 8 numeral system. Octal number's digits use the symbols 0 to 7. Each digit represents a power of 8. Octal is less common than binary or decimal but can be useful for representing computer memory addresses efficiently.
Hexadecimal
Hexadecimal number is a number expressed in the base 16 numeral system. Hexadecimal number's digits use 0 to 9 and the letters A to F (case-insensitive) to represent values from 10 to 15. Each digit represents a power of 16. Hexadecimal is commonly used in computer science for representing memory addresses and color codes.