iCloud

 iCloud  is what Apple summarily calls all of its various online services such as Apple ID, online storage, syncing, as well as a silo of sorts with iCloud identifiers and small group photo sharing.

Status
You can check the status of various Apple online services here:
 * http://www.apple.com/support/systemstatus/

2022-03-21

 * https://twitter.com/BrandonButch/status/1505952600851636228
 * "Looks like iCloud and other Apple services are down. Having issues with iMessage and signing in to iCloud 🧐" @BrandonButch March 21, 2022
 * 2022-03-21 The Verge: An Apple outage is affecting Apple Music, iCloud, and other services / Several of Apple’s cloud services seem to be having issues right now
 * https://twitter.com/MacRumors/status/1505951324252254208
 * "iCloud is Down and Other Apple Services Experiencing Issues https://www.macrumors.com/2022/03/21/icloud-and-apple-services-down/ by @rsgnl" @MacRumors March 21, 2022

Your iCloud storage is almost full
Recently (2016?) Apple seems to have started sending lots of fear-inducing emails with the subject line: "Your iCloud storage is almost full"

Have received these emails recently (in 2016):
 * undefined

Photos
iOS 9.x has a UI in the Settings app (iCloud option) for enabling various iCloud Photo features:



Backup
iOS 9.x has a UI in the Settings app (Storage option) for enabling various backup features, which use iCloud:



Note: the "Photo Library" backup option is different from the "iCloud Photo Library" and "My Photo Stream" options in the explicit iCloud Photos preferences above.