Leonie,
As I understood it, in iPhoto the Events "library" was where the master copy of your photos was located and if you deleted a photo from a folder or an album, it would still exist in Events, or if you edited a photo, you could always revert back to the original because the original version was stored in Events. However, once you deleted a photo in Events it would disappear completely from Events, Folders and Albums. Was this correct?
I have a huge library so until I have a better grasp of how the iCloud Photo Library works (which in my case will most likely take months) I will continue to store my Photos library on an external hard drive. So when I sync each device using a USB connection I download all new photos from the Camera Roll to my external hard drive via my Mac.
In the new Photos app on my Mac, is there a "Library" that houses a master copy of my photos? In other words, where can I delete a photo from my Mac or any of my iOS devices without losing the original copy? On my iPad when I go to Recently Deleted it gives me a countdown until deleted photos will be removed from all devices. I think of devices as iOS, as opposed to my Mac OS. From your response above it seems that even if I'm not using the iCloud Photo Library my Mac is considered just another device and any Photo Stream deletions eventually become universal and permanent. If those photos were previously imported to Photos on my Mac, will they completely disappear there as well, or will an original still exist on my Mac OS Photos app?
I have Yosemite on my Mac and iOS 8.4 on my iPhone 6 and my iPad Air. Thanks!