Photo Stream only adds new photos as they are taken. You won't see a bunch of photos just because you turned it on. You'll have to take or add some photos to the Photos app on a device that has the the same iCloud account, and Photo Stream turned on.
Photo Stream is not a shared album between devices. It is a quick way to see and save photos from other devices. There is also a limit to how many, and how long photos are stored in Photo Stream.
For permanent shared albums, use iCloud Photo Sharing.
If you use iCloud Photo Library on all your devices, Photo Stream becomes redundant.
If Photo Stream seems kind of limited, it's because it is the oldest of the three photo features mentioned. It was pretty cool when it first came out, but due to how both the iOS and OS X apps have progressed it's not as useful as it used to be.
