A number is a numeral that represents a quantity. The quantity can be anything, like the number of apples in your garden, the number of children in your family, the distance to our next destination, or the amount of money you have in your account.
A decimal is a numeral that has a place value and uses a base-10 positional notation system. Decimal is the number system most commonly used by humans. It consists of ten digits: 0,1,2,3,4,5,6,7,8 and 9.
Binary is a numeral that has two digits and uses a base-2 positional notation system. Binary is the number system that most computers use because it is the simplest. It only consists of two digits: 0 and 1.
Hexadecimal is a numeral that uses a base 16 positional notation system. Hexadecimal is a way to write binary numbers in an easier way for humans to understand. It uses sixteen symbols or letters (0-9 and A-F).