What is difference between iCloud Photo Library and my Photo Stream?

two totally different services

this may help - iCloud Deconstructed

the basic difference is that ICPL is a syncing service so all changes made on any connected device are made on all devices and your photos can be seen on iCloud.com -- iCloud Photo Library - Apple Support

MyPhotoStream is a streaming service that streams photos one time between devices when they are first added - no edit or changes are streamed and you can not see the photos on iCloud.com -- My Photo Stream - Apple Support

Two different services for two different purposes with two different results

LN

As far as I can tell, the difference is iCloud Photo Library, stores your photos in iCloud, and Photo Stream shares your photos across your devices you are logged into with your Apple ID.

For more information, read here: About iCloud Photo Library and My Photo Stream - Apple Support