The current system (Photo Stream) only stores your photos (up to 1000) on iCloud for 30 days.
The new system will let you store your photos on iCloud indefinitely. If you have a 70GB photo library on your Mac, you can store all 70GB of photos on iCloud, and access any picture from any of your devices. So if you have a 16GB iPhone, you're obviously not storing all 70GB of your photos on it, but you'll be able to look at the 70GB of pictures you've stored in iCloud, and pull any of them down that you want.