overview

Relationships define how two entities relate to each other.

public class Blog
{
    public int Id { get; set; } // Primary Key
    public string Name { get; set; }
    public virtual Uri SiteUri { get; set; }

    public ICollection<Post> Posts { get; }
}

public class Post
{
    public string Title { get; set; }
    public string Content { get; set; }
    public DateTime PublishedOn { get; set; }
    public Archived { get; set; }

    public int BlogId { get; set; } // Foreign Key
    public Blog Blog { get; set; }
}

Above:

The Blog.Posts property connects Blog to Post.
The Post.Blog property connects Post to Blog.
This connection is said to be a one-to-one relationship.
These properties are said to be navigations.
Note that these two navigations result in one relationship.