iCloud Photo Library
The iCloud Photo Library will store all your photos and personal videos and has no limits, as long as you have the iCloud storage space available (you may end up having to purchase more storage to accommodate your photos and videos). The iCloud Photo Library is accessible from your iPhone, iPad, Mac and even on a Windows PC by logging into iCloud.com. The iCloud Photo Library can store full resolution photos and supports the most common photo types including jpg, RAW, png, and gif.
Photo Stream
Photo Stream is great because it does not count against your storage you can stream up to 1000 photos or the last 30 days of pictures. Photo Stream is compatible across all devices including iPhone, iPad, iPod, and Mac. You can store the optimized version of your photos on your devices, meaning the the viewable quality of the photo on that device may be reduced, but the full resolution photo is still in the cloud and it will require less storage on that device.