Photo Stream differs slightly from iCloud Photo Library. Instead of storing all your images it gives you a way to quickly access the last 1,000 you took or those taken within the last 30 days via all your devices. If you don’t want to use iCloud Photo Library because of the space it uses in your iCloud then it’s a good idea to use Photo Stream instead, as it takes up no space in your iCloud library.
First, you must manually enable it on each machine you want to use it with. On the iPhone, open Settings, tap Photos & Camera and enable the Photo Stream option.
Photo Stream doesn’t keep images forever, so if you do want to save them permanently on other iOS devices (like an iPad), you’ll need to add them to the Camera Roll. However, your pictures are saved permanently on the device you used to take them so they will be saved to Camera Roll on your iPhone.
Photos will only transfer to other devices if your iPhone has access to a Wi-Fi connection.