实体框架中的导航属性有什么作用？

Question

I use EF6 and these are my POCO's.

public class Author
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

public class Book
    {
        public int Id { get; set; }
        [Required]
        public string Title { get; set; }
        public int Year { get; set; }
        public decimal Price { get; set; }
        public string Genre { get; set; }

        // Foreign Key
        public int AuthorId { get; set; }
        // Navigation property
        public Author Author { get; set; }
    }

1. What is the point of the navigation property Author ?
2. What is the point of making the property virtual ?
3. Are navigation properties specific to Entity Framework?

EDIT: after a bit more research I've found this thread , which nicely answer my first question.

Can people help with the last two questions please?

Answer 1

2: if you do virtual navigation properties then will work lazy load value of this property:

public class Book
{
    //...
    public int AuthorId { get; set; }
    public virtual Author Author { get; set; }
}

It work: EF create proxy to your entity (Book) and overrides navigation property (Author). When you load entity (Book) of the database value of the navigation property (Author) is not load. Only the first get value of navigation property (Author), it will be load ( http://blogs.msdn.com/b/adonet/archive/2009/12/22/poco-proxies-part-1.aspx ).

3: inherently navigation properties is no specific to Entity Framework. This link to other entities. But term "navigation properties" is specific to EF.