iCloud Photo Library is a Cloud-based storage facility for your photos which can be accessed directly by your devices running iOS 8 and a Mac running Yosemite.
However, you also must be running the Photos Application in order to use the iCloud Library.
If you like the way that iPhoto worked for you, you can turn off iCloud Photo Library on all of your devices and Mac, and re-open iPhoto by following these instructions:
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I'm still using iPhoto and Aperture, but I am also playing around a lot with Photos. I automatically import my Photo Stream to iPhoto, but I also will manually import photos into Photo just so I have them there as well if I want.
I have not used (and do think I will use) iCloud Photo Library. I like keeping my photos on my Mac and on my backups like Time Machine. I just don't want my stuff being so easily out there in cyberspace for someone to grab if they wish....
Hope this helps,
Cheers,
GB
BTW - turning off iCloud Photo Library does not stop your iOS backps to iCloud from continuing to back up your photos for safekeeping in the case where you might need to restore your iOS device. Any photos on that backup will be fully restored as they are today.