Whats the need to incude other class as member of class [Resolved]

Here the Genre and Artist are two different classes.
In Album class both classes are included i.e.;line number:9 and 10
public Genre Genre{get;set;} and
public Artist Artist{get;set;}
Whats the use of thsi classes inside Album class.

Please help:

1. public class Album
2. {
3. public int AlbumId { get; set;
}
4. public int GenreId { get; set; }
5. public int ArtistId { get; set; }
6. public string Title { get; set; }
7. public decimal Price { get; set; }
8. public string AlbumArtUrl { get; set; }
9. public Genre Genre { get; set; }
10. public Artist Artist { get; set; }
}

[Resolved]