For you recommends content to you based on your interactions with Glo's content. Library does not recommend content to you. Library records your activity on the platform, from your class history to your course purchases, to the teachers that you Follow. Essentially Library is more controlled by you, you can add classes to your Library, Favorites and it reflects the classes, programs and collections you are participating in.
For you displays some content that is in your library on its various shelves such as Recently Practiced and Your current programs.