Find My iPhone, iCloud Backup, iCloud Drive, iCloud Keychain, iCloud Mail, iCloud Notes, iWork for iCloud, Photos, News, and more are all unavailable for an unspecified number of users.
The outage has been going on since approximately 8:00 a.m. Pacific Time, so if you've been experiencing problems with iCloud services, this is why.
It's not clear when the issue will be resolved, but Apple often addresses these problems quickly. We'll update this post when all of the iCloud services have been restored to working order.
Update: According to Apple's System Status page, the iCloud outage has been fixed.