The cloud is a metaphor referring to networks, specifically the Internet, that have been represented in engineering and network drawings for some time. In reality, the cloud is the latest iteration of a concept that has been called many things in the past including ASP (application service provider), on-demand, utility computing and as a service (Software as a Service, Infrastructure as a Service).
Cloud technologies are simply this:
A pool of computing resources (servers, storage, applications, and voice services) that is provided as-needed to businesses from a provider’s network, eliminating the need for on-site equipment, maintenance, and management. Cloud technology technologies enables IT departments to increase or add capabilities as needed without purchasing equipment and software, training employees to support it, and using office space, power, and cooling to house it.
They provide end users immediate access to new, always-on features from nearly any device in any location. They also provide the business a predictable, subscription-based, pay-per-use way to fund IT.