The new Photos app (part of OS X 10.10.3) replaces iPhoto... the photo management app that we all grew to love and made part of our personal (dare I say professional?) workflow. Photos is a complete redesign of how we import, manage, edit and share our photos. Personally, I really like the clean interface, and because it feels much like the Photos app on iOS, I felt comfortable with it the first time I launched the app on my MacBook Pro.
Having said that, there are some things about Photos that some users don't appreciate... yet. The interface, for example is one of those things. In iPhoto, there was a Sidebar that gave you a listing of all your albums, Smart Albums, projects, iCloud subscriptions, etc. In an effort to simplify the interface, Apple decided to remove the Sidebar from Photos, but rest assured, it's not completely gone. Here's a simple trick to get it back.
First, we launch the Photos app. Next, we go to View | Show Sidebar (Command+Option+S).
That's it! Now that the Sidebar is enabled, you have full access to your content like you did in iPhoto.
I personally wouldn't get too attached to using the Sidebar. If Apple felt it was important enough to hide it by default in this release of Photos, there's a possibility that they will get rid of it altogether in a future release, so it's good to prepare yourself for that early.