Shoot a video on your iPhone, take a picture with your iPad, import from your DSLR on your Mac, and all of it goes up to Apple's servers and is made available on all your other devices. To accomplish this, iCloud Photo Library works with Photos for Mac as well as with Photos for iOS and, as the glue that holds everything together.
ICloud Photo Library aims to deliver on the promise of having all your photos available on all your devices all of the time.