The recent OS X 10.10.3 update includes Apple’s new Photos app, their eventual replacement for iPhoto and Aperture. Immediately following the release of OS X 10.10.3, iPhoto and Aperture were both removed from the App Store.
iPhoto and Aperture users who are thinking about upgrading an older version of OS X to Yosemite should be warned… Yosemite will not run older versions of iPhoto and Aperture, and you can no longer update them from the Updates tab of the App Store because they’ve been removed. New converts to Yosemite will be forced to use the Photos app, or move to a non-Apple solution.
If you have previously purchased iPhoto or Aperture on an older version of OS X, and need to reinstall it for that version of OS X, you can still re-download the full applications from the Purchases tab of the App Store. If iPhoto or Aperture came bundled with your version of OS X, and was never linked to an Apple ID by updating it, reinstallation is no longer an option via the App Store.