Cloud computing is a type of computing that relies on sharing computing resources rather than having local servers or personal devices to handle applications.
In cloud computing, the word cloud (also phrased as “the cloud”) is used as a metaphor for “the Internet,” so the phrase cloud computing means “a type of Internet-based computing,” where different services – such as servers, storage and applications — are delivered to an organization’s computers and devices through the Internet.
Cloud computing is comparable to grid computing, a type of computing where unused processing cycles of all computers in a network are harnesses to solve problems too intensive for any stand-alone machine.
source :webopedia