Apple introduced Photo Stream a few years back. It’s a great idea and is implemented well, but I’ve come to realize many people have misconceptions about the service.
Photo Stream Works Via Wi-Fi
Your iOS device must connect to a wireless network (Wi-Fi) to send your photos to Apple’s iCloud service.
Photo Stream Needs iCloud
Photo Stream synchronizes photos to other devices using iCloud. These can be other iOS devices, Macs, or Windows PCs. Each device or computer must be configured properly. Macs and iOS devices come with the iCloud software. Windows PCs must download and install the iCloud Control Panel.
Photo Stream is Not a Backup Service
It keeps your photos from the last 30 days. Apple feels that is enough time for other configured devices to download those pictures.
Photo Stream Enables You to Backup Your Photos (It’s Not a Backup Service)
This is best explained using a common scenario: You take a picture with your iPhone. That picture will remain on your iPhone until you delete it. If you have iCloud configured on that iPhone, it will upload the photo automatically to iCloud when the phone connects to a Wi-Fi network. Once the photo is in iCloud, it will download to all computers and devices configured to receive it. Now that the photo is on your computer, you may back it up (you do that, right?!) and you can either leave it on your phone or delete it from your phone.
Learn More: My Photo Stream FAQ