Sum ( ) , it is a function which return the aggregate sum of any expression , like salary of multiple employee , sum of population in any particular area etc .
Syntax :

select sum ( Expression ) from student where clause ; // expression can be numeric value or any mathematical formula 

Example :
Table name : Employee_Info

  Employee_name            Employee_Age          Employee _Salary
   Mukesh                  23                           100000
   Ayush                   24                           200000
   Ishan                   20                           400000
   Pranav                  35                           700000
   Abhishek                26                           800000
   Ravi                    25                           300000
   David                   40                           800000

Suppose you want to retrieve Sum of Employee Salary whose salary is greater than 400000 .

select sum ( Employee_Salary ) as " Total Salary " from Employee_Info where Employee_Salary > 400000

Output :

 Employee_name            Employee_Age          Employee _Salary
     Pranav                  35                      700000
     Abhishek                26                      800000
     David                   40                      800000

In this result if two employee have same salary then both will come in Output . As you can see above example
If you want to select distinct employee then you can also do , How ? see below :

select sum ( distinct Employee_Salary ) as " total salary " from Employee_Info where Employee_Salary > 400000

Output :

Employee_name            Employee_Age          Employee _Salary
    Pranav                  35                       700000

In above result you can see , it return only distinct salary .
Sometime we have to use Group By statement with Sum function because sometime we have a table in which two or more department are same and we have to group them and then calculate the result . For example see below :
Table name -> Branch_Department

        Department      Sales
        Mechanical      360
        Electrical      100
        Electrical      110
        Electronics     150
        Electronics     170

In above example you want to sum of different department then you can do this by Group By statment :

select Department , sum ( Sales ) as " total sales " from Branch_Department Group By Department ;

Output :

                    Department            Sales
                     Mechanical           360
                     Electrical           210
                     electronics          320