We have learnt about natural and whole numbers in the previous chapters. But when we subtract a greater whole number from a smaller whole number, then the difference will not be a whole number. Hence, there is a need to extend the number system to include such numbers. Their are many examples in hour day to day life related to such type of numbers.
(i)    During the winter season, the minimum temperature in Shimla may be 5°C below 0°C. 
So we represent this as –5°C, read as minus five degree centigrade.
(ii)     the depth of an ocean, say 300 metres below the mean sea level, can be expressed as –300 metres high. If height is considered as positive, depth is considered as negative height and vice versa. 

Their are infinite numbers with negative sign on the numebr line. All these numbers are less than zero and are called negative numbers. They are –1, –2, –3.  
Whole numbers along with the negative numbers are called integers.
Their are some important properties with integers.
(i)    There are infinite positive numbers to the right of zero and there are infinite negative numbers to the left of zero as well.
(ii)    In the number line of integers, every number on the right is greater than all the numbers on its left.
(iii)    Zero is neither negative nor positive.
The natural numbers 1, 2, 3, ... , etc., in the system of integers are called positive integers. The numbers to the left of zero are called negative integers and are always denoted as –1, –2, –3, ... , etc. . The whole number 0 is neither a positive integer nor a negative integer.