IaaS – Infrastructure-as-a-service. A public cloud or co-location service in which IT infrastructure equipment; compute, storage, and networking, is supplied  on demand, on a pay-as-you-go basis. Each resource can be offered as an individual service. Users install their own software and applications on such infrastructure and no longer have the necessity of provisioning their own servers, storage arrays and network boxes. PaaS (Platform-as-a-Service) is an extension of this to add system software.