Apple defines iCloud as follows:
“iCloud securely stores your photos, videos, documents, music, apps and more — and keeps them updated across all your devices. With iCloud, you can easily share photos, calendars, locations and more with friends and family. You can even use iCloud to help you find your device if you lose it.”
iCloud, being an Apple product, works best on Apple devices, and is really easy to set up. However, you can also access your iCloud account on Windows and Android devices, but in a more limited way.
Read Apple’s full description of iCloud, and its features, here.