iCloud is what Apple summarily calls all of its various online services such as iCloud identifiers, online storage, syncing, etc.
You can check the status of various Apple online services here:
Your iCloud storage is almost fullRecently (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):
iOS 9.x has a UI in the Settings app (iCloud option) for enabling various iCloud Photo features:
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.