Difference between summation and integration

The use of Summation arises when there is need of discrete sum of quantities( i.e. addition of few distinct numbers or even large series);
Integration is used when the addition is not limited to few or more discrete quantities but the addition is to be done on a continuous basis.
In short, Summation= Discrete addition :Integration= Continuous addition.

This area can be given as the sum of much smaller areas included in the bounded area. But Summation involves the discrete values with the upper and lower bounds, whereas the integration involves continuous value.
Summation is a process of discrete addition while Integration is a process of adding continuous things.