Here are some things to know.
— To get it, go to the Mac App Store and check for updates. You'll need the new version of the Mac operating system, 10.10.3. On phones and tablets, you need at least version 8.0 of iOS, though 8.2 is recommended. You can also access your photos on Windows computers with Apple's free iCloud for Windows software. (Sorry, it's not available for Android.)
— If you don't see prompts to turn on iCloud Photo Library, go to "Preferences" on the Mac Photos app and check "iCloud Photo Library." On iPhones and iPads, go to "Photos & Camera" in the settings. In both cases, choose the option to optimize storage to save space on your device.
— Why optimize? That will always keep the full-resolution version online, and what you see on your device will depend on how much space you have left. If it's running low, the Photos software will replace full-resolution images with smaller files, particularly for older shots. They will be fine to view on that device, but you might notice degradation when you zoom or crop. As long as you're online, the software will download originals automatically when you need them, such as for printing.
— Photos on the Mac gives you the option to store photos and videos in folders, but the preferred way is for Photos to import copies into its library. You need to keep this default option for iCloud Photo Library to work. But you will lose control over organization, as Photos takes over managing that for you.
— The 5 gigabytes of free storage on iCloud won't get you far. You can get a total of 20 gigabytes for $1 a month, 200 GB for $4, 500 GB for $10 and 1 terabyte for $20.
— When you delete a photo on one device, it will delete across all of your others as well. But, the new Photos app gives you a warning first and then about a month to retrieve the image from iCloud. Previously, photos deleted from iPhones and iPads disappeared immediately — bad for parents with small children who like to haphazardly delete things.
— If you change your mind and want to stop using iCloud Photo Library, you'll be able to retrieve all your originals first.