Mac OS X 10.10.3, currently in beta as of this post, comes with a new Photos app. This Photos app is replacing Aperture, and when OS X 10.10.3 is released Aperture will no longer be sold on the Mac App store. Existing Aperture owners will still be able to use the app, but new licenses will no longer be available.
Many Aperture users who have played around with the new Photos app don’t have great things to say about it. It lacks features that made Aperture suited for professional and hobbyist photographers, and Photos appears very basic in comparison.