Rackspace Cloud is a collection of cloud computing services for building, hosting, and managing cloud-based infrastructures. Rackspace’s "Cloud Servers" infrastructure service enables users to instantly ramp up or scale down any number of virtual servers as needed, providing elastic cloud-based infrastructures with high levels of availability. This service includes a Cloud Servers API lets users easily build, configure, and manage VMs. Meanwhile, Rackspace’s "Cloud Sites" PaaS (Platform-as-a-service) enables application developers to build out complex applications without having to worry about underlying infrastructure and provides them with compute, storage, and bandwidth resources, unlimited databases and sites, and reseller capabilities for client billing and technical support. Rackspace Cloud offers additional services and tools designed to manage cloud security, build APIs, and create databases and other infrastructure components. Rackspace bills users on a utility computing basis.