Photos is a photo library manager application made by Apple, for both macOS and iOS. You can learn about its features here. Although it can be used independently of iCloud, the focus here is how it fits into iCloud.
On a Mac, you’ll find the Photos app in the Applications folder. The Photos app is dependent on an accompanying Photos Library, which will be located in your Home folder.
Using the Photos app to manage your photos is a lot easier than manually organising photos. Just import all your photos, and the Photos app provides a multitude of methods to view, find, and organise images.
Problems arise when you have different photos on different devices, and need to consolidate, and have access to them on all your devices. iCloud Photos solves this problem.
Any photos or videos you add to your Photos library on your Mac, iPhone and iPad will be added to your iCloud Photos library.
The result is, all your photos in one place, iCloud. And, as they’re stored in iCloud, all your devices can download a copy of the library. If a device doesn’t have enough storage space, then it will store low quality versions, with the full quality versions available ‘on demand’. And, any changes you make to a the library will be synchronised with your other devices.
Enable iCloud Photos (macOS)
Download Originals, or Optimise Storage? (macOS)
If your Mac has limited storage space, iCloud Photos has a clever option, Optimise Mac Storage (see above screenshot):
“When your Mac is low on space, full resolution photos and videos are automatically replaced with smaller, device sized versions. Full-resolution versions can be downloaded from iCloud at any time.“
Ideally, we wouldn’t need to use this feature, and would instead select Download Originals to this Mac. (This makes sense especially if using Time Machine to backup the computer – if those full-resolution photos aren’t stored on the Mac, they can’t be backed up.)
Enable iCloud Photos (iOS / iPadOS)
Download Originals, or Optimise Storage? (iOS / iPadOS)
The same options are available on iOS / iPadOS. As mobiles devices tend to have less storage, it’s understandable to use the Optimise Storage option.