iOS 8 introduced iCloud Photo Library which removes some of the older restrictions on Photo Stream, including the most recent 1,000 photos or the last 30 days of your photos, whichever is greater. Instead, as long as you have the iCloud storage to support it, you can keep saving photos. If you have iCloud Photo Library enabled, the traditional form of Photo Stream is replaced and all your photos are synced to iCloud Photo Library.
However, if iCloud Photo Library is not enabled, you can continue to use Photo Stream in its original form where your Camera Roll and normal Photo Stream exist the same way, and are still subject to the same 1,000 photo limit.
iOS 7 brought with it true shared Photo Streams that allow you and your friends to share albums through iCloud. You can also like and comment on photos and allow your friends to add them as well. Shared Photo Streams, now referred to as iCloud Photo Sharing, are extremely useful for times when you go on a group trip and everyone has photos to share. Just create a shared Photo Stream and add your friends. You'll then get notifications when they comment on your photos or add some of their own. These show up under the Shared tab in the Photos app on iPhone and iPad and under the iCloud section of iPhoto on the Mac.