Interfaces describe what an object can do. Abstract classes describe what an object is.

interface IPet 
{
    void Eat();
    void Play();
    void Sleep();
}

public class Dog : IPet 
{
    public void Eat() { … }
    public virtual void Play() { … }
    public void Sleep() { … }
}

Virtual methods define implementations, but those implementations can be overridden in derived classes.

public class Labrador : Dog 
{
    public override void Play() { … }
}

Abstract classes can contain both implemented and non-implemented members.
Any class that contains an abstract method must also be declared abstract.