For many years user data has been computer-centric, a major floor with this approach is that is that if the computers' hard drive fails without a backup you're up the proverbial creek without a paddle as the data is lost with the hard drive. A second issue with computer-centric data storage is that it can be tricky to access your data when not at your computer.
Cloud computing solves these and many other issues by storing data online (in the cloud) affording access to data securely and in sync across multiple devices. All your data, everywhere, all the time.
How cloud computing works
Is iCloud Cloud Computing? Technically yes... iCloud is a cloud service provided by Apple, iCloud offers a variety of services in the cloud such as Mail, File Storage, Calendar, Contacts to name just a few. Other cloud service include One-drive, Dropbox, Box and Google Docs.
Here's how Wikipedia describes cloud computing: Cloud computing is a type of Internet-based computing that provides shared computer processing resources and data to computers and other devices on demand. It is a model for enabling ubiquitous, on-demand access to a shared pool of configurable computing resources (e.g., computer networks, servers, storage, applications and services), which can be rapidly provisioned and released with minimal management effort. Cloud computing and storage solutions provide users and enterprises with various capabilities to store and process their data in third-party data centers that may be located far from the user–ranging in distance from across a city to across the world.