Cloud computing provides on-demand access to virtualized IT resources that users can share or subscribe to. It allows configurable resources to be easily provided from a shared pool of networks, servers, storage, applications and services. There are three main layers of cloud computing: Infrastructure as a Service (IaaS) provides hardware resources, Platform as a Service (PaaS) provides development platforms, and Software as a Service (SaaS) provides applications for users. Cloud services can be deployed through private, public or hybrid clouds and are measured based on usage to allow users to pay only for what they use. Security is a major consideration in cloud computing across areas like identity management, access control, authorization and data transport.