Shadowing: It hides a base class member in the derived by using the "New" keyword.
It is used to explicitly hide a member inherited from base class.
new and override both cannot be used for the same member.

eg:-
class Employee
{
public virtual double CalculateSalary()
{
return basic_salary;
} 
}
class SalesPerson: Employee
{
double sales,comm;
public new double CalculateSalary()
{
return basic_salary+(sales*comm);
}
}
static void main(string[] args)
{ 
SalesPerson sal=new SalesPerson();
double s=sal.CalculateSalary();
Console.WriteLine(s);
}

Overriding:In this methods have same names,same signatures,same return types but
 in different classes.C# uses virtual and override keyword for method overriding.

eg:-
class Employee
{
public virtual double CalculateSalary()
{
return basic_salary;
} 
}
class SalesPerson: Employee
{
double sales,comm;
public override double CalculateSalary()
{
return basic_salary+(sales*comm);
}
}
static void main(string[] args)
{ 
Employee sal=new SalesPerson();
double s=sal.CalculateSalary();
Console.WriteLine(s);
}

When you call the derived class object with a base class variable.In class of 
overriding although you assign a derived class object to base class variable it will 
call the derived class function. In case of shadowing or hiding the base class
function will be called.