DDD Book, Eric Evans: What is meant by "The FACTORY should be abstracted to the type desired rather than the concrete class(es) created."?

I think it means you should never return a concrete type from your factory.

For example, if you have an interface, let's say ISomething, an abstract class SomethingBase and finally some classes that implement this interface and inherit from the base class. Your creation method should return the interface type, instead of the base type. I think that is the idea.

public ISomething Create() { ... }

Instead of

public SomethingBase Create() { ... }

I hope it helps, Carlos.

Carlos Loth's answer is correct, but you should always remember to use an Abstract Factory as well, as this will allow you to couple concrete factories to concrete types without coupling consumers to concrete factories or types.

public interface ISomethingFactory
{
    ISomething Create();
}

public class SomethingFactory : ISomethingFactory
{
    public ISomething Create()
    {
        return new Something();
    }
}