The new Photos app launched as part of OS X Yosemite 10.10.3. While the app is considered a 'vast improvement' over iPhoto, Aperture users will miss some of the professional image editing features they're accustomed to.
If you're an Aperture user you can still use the app and access it from your list of purchases; however, it will no longer be updated and you may want to consider a switch to Lightroom. Adobe updated Lightroom with an integrated Aperture and iPhoto import utility as of version 5.7.
If you've tried the new Photos app, let us know what you think of the application in the comments!