Explain Abstract class ?

Abstract class has following features :

1. cannot be instantiated
2. contain abstact methods and properties
3. cannot modify abstract class with sealed modifier
4. non abstract class which inherits from abstract class must implement inherited abstract methods and properties
5. An abstract class that implements an interface might map the interface methods onto abstract methods

abstract class Shapes
{
abstract public int Perimeter();
}

class Square : Shapes
{
int x;
public override int Perimeter()
{
return 4x;
}
}