Cloud computing allows on-demand access to shared computing resources like servers, storage, and applications via a network. It has three service models - Software as a Service (SaaS), Platform as a Service (PaaS), and Infrastructure as a Service (IaaS) - and four deployment models - private cloud, community cloud, public cloud, and hybrid cloud. However, data security and privacy concerns remain barriers to cloud adoption according to surveys of IT managers and CTOs. Advanced issues in cloud security include abstraction, lack of execution controls, third-party control of data, and multi-party processing.