Relation between generic type parameter and generic type definition is known as variance.

Covariance and contravariance are extensions for generic type parameters for arrays, delegates and interfaces.

1.COVARIANCE-  In Covariance 'out' keyword is used to declare input parameter. It keeps the work compatible. In covariance, where IEnumerable<object> is expected  we can use IEnumerable<string> in place of that.

e.g of covariance:-

public interface IEnumerable<out T> 
{ 
IEnumerator<T> GetEnumerator(); 
} 
public interface IEnumerator<out T>
 { 
   T Current { get; } 
   bool MoveNext(); 
}

2.CONTRAVARIANCE:- The functions of contravariance are just different from covariance. Input parameters are passed by using 'In' keyword.

e.g of contravariance:-

void DoSomethingToAFrog(Action<Frog> action, Frog frog) 
{ 
action(frog); }
 ... Action<Animal> feed = animal=>{animal.Feed();} 
 DoSomethingToAFrog(feed, new Frog());