Cloud computing is the delivery of computing as a service rather than a product, whereby shared resources, software, and information are provided to computers and other devices as a metered service over a network (typically the Internet).
Computing clouds provide computation, software, data access, and storage resources without requiring cloud users to know the location and other details of the computing infrastructure.
End users access cloud based applications through a web browser or a light weight desktop or mobile app while the business software and data are stored on servers at a remote location. Cloud application providers strive to give the same or better service and performance as if the software programs were installed locally on end-user computers.
Cloud computing on Wikipedia