OS X Yosemite: Switch to Photos from iPhoto
When you update to OS X v10.10.3, the all-new Photos app is installed automatically. It was engineered from the ground up for the way you take and organize photos today.
When you first open Photos on your Mac, your photo library is upgraded. The upgraded library is compatible with both Photos and iPhoto (if installed), but any changes you make in Photos won’t appear in iPhoto, and vice versa.
If you previously had iPhoto on your Mac, it’s still on your Mac. You can open it from Launchpad or from the Applications folder in the Finder. However, it’s strongly recommended that you use Photos instead of iPhoto.