Photo syncing explained
Apple now has three separate but related photo syncing and sharing services that all rely on iCloud. iCloud Photo Library is the most comprehensive: it syncs your entire photo library to iCloud. If you have multiple devices signed in with the same Apple ID, they all sync to the same online library, so all your photos from all the devices appear online and on every device, although the syncing process automatically eliminates exact duplicates. This uses your iCloud storage quota, so unless you have a very small library, you may need to pay a monthly fee for extra online space.
My Photo Stream is a more limited service that only uploads photos taken in the last 30 days, up to a limit of 1,000 photos. It doesn’t count against your iCloud storage limit. The third service is iCloud Photo Sharing. This doesn’t count against iCloud storage either, and is limited to 5,000 photos per shared album. You can use this to upload photos to a cordoned-off area of iCloud that is shared with people that you invite. All are enabled in iCloud’s preferences under Photos on a Mac, and in Settings > Photos & Camera on iOS devices.