When Apple released iTunes 12, they got rid of the useful sidebar that displays media libraries, devices, playlists, and more. I explained how to restore it, sort of.
The Photos app, recently released as part of the OS X 10.10.3 update, hides the sidebar by default. If you used iPhotos, the sidebar was a practical way of viewing a lot of top-level information about your photo library. For example, it displays entries for All Photos, Last Import, and your photo albums.
To display the sidebar in Photos, choose View > Show Sidebar, or press Command-Option-S. Your sidebar will display like this:
You can hide it again using the same menu item (now Hide Sidebar) or keyboard shortcut if you wish.