Photos for OS X is now available as part of the OS X Yosemite 10.10.3 update. Photos is intended to replace both iPhoto and Aperture. It does the former well but falls a bit short as the replacement for Aperture. Photos for OS X attempts to address three important photo library needs. It is used to organize your photos so you can find that one photo you want to show easily and it contains tools to edit photos to make corrections to their appearance. Of course, it also allows you to share photos, make photo albums, slideshows, and prints. Photos also integrates with the Photos app on all your devices with the goal of connecting your iPhone, iPad, Apple TV and Apple Watch all to the same photo library.
Photos for OS X now mostly follows the organizational pattern of the Photos app on your iOS devices. The app opens with your photos grouped into Moments and Collections. Moments organizes the photos by age and location whereas Collections is at a bit higher level. Let’s say you took a trip to China. Collections might be all of the photos from that trip while Moments would be photos from your time on Yellow Mountain.
The editing tools in Photos are not as good as Aperture but far better than iPhoto. You can take a marginal shot and improve it by adjusting exposure, cropping, straightening, etc. You have some detailed adjustments you can make such as sharpen, noise reduction, white balance, and levels as well as the standard brightness and contrast.
Perhaps the biggest change is iCloud Photo Library. You can have your entire Photo library stored in iCloud for effective syncing and access from any of your devices. One word of warning, however: if you have a big Photo library you may reach the limit of your free 5GB of iCloud storage but Apple is happy to provide you more for a few dollars a month. If you do not need or want all of your Photos on all your devices you can toggle iCloud Photo Library off.
Performance-wise, I find Photos to be considerably faster than iPhoto and the interface seems more intuitive.