Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction. Cloud computing is a significant advancement in the delivery of information technology and services. This paper explains the security threats and security mechanism in cloud computing and outlines what are the major security concerns which are stopping the organization from moving completely to cloud.