Cloud computing is a particular implementation of distributed computing. It inherited many properties of distributed computing such as scalability, reliability and distribution transparency. The transparency middle layer abstracts the underlying platform away from the end user. Virtualization technology is the foundation of Cloud computing. Virtual machine provides abstraction of the physical server resources and securely isolates different users in multi-tenant environment. To the Cloud services consumer, all the computing power and resources are accessed through high speed internet access by client platforms. This eliminates the cost to build and maintain local data center. Resource pooling and rapid elasticity are the main characters of Cloud computing. The scalability of Cloud computing comes from resources which can span multiple data centers and geographic regions. There is virtually no limitation on the amount of resources available from Cloud. New processing and storage resources can be added into the Cloud resource pool seamlessly.