Cloud Computing is a model for delivering information technology services in which resources are fetched from the internet rather than a direct connection to a server .Cloud computing as its name such as, suggests store the accessed information in the “cloud”, and doesn't require a user to be in a particular place to gain access to it. So it saves managing cost and time .Cloud Computing enables organizations to consume compute resources as a utility ---just like electricity--- rather than having to build and maintain computing infrastructure in-house. Many industries, such as banking, healthcare and education are moving towards the cloud due to the effectiveness of services provided by the pay-per-use pattern based on the resources such as processing power used, bandwidth consumed, data transferred or storage space occupied ,etc .IT organizations have expressed their anxiety about security issues in cloud computing .It is the most important challenge in cloud computing .This research paper outlines what cloud computing is, the various models of cloud and the important security risks and issues that are currently present within the cloud computing industry .This paper also analyzes the key challenges that the cloud computing is facing.