Cloud Computing Is an Internet Based Computing Platform Which Is Shared Information, Hardware and Software Resources, Is Provided to Computers and Devices On-Demand, Like the Power Grid. It Is a Great Idea to Make Many Standard Computers Together to Get a Super Computer Where It Can Do a Lot of Things Powerfully. It Aims to Construct a Perfect System With Powerful Computing Capability Through a Large Number of Relatively Low-Cost Computing Unit, and Using the Innovative Business Models Like Saas, Paas, and Iaas to Distribute the Powerful Computing Capacity to End User’S Hands. This Paper Provides a Snapshot of Cloud Computing Background, Classifications, Service Model, Existing Security Issues and Typical Security Measures.