Answer Description

Infrastructure as a service (IaaS) is a cloud model where a company leases infrastructure services ( server, networking, storage, virtualization..) from a service provider while maintaining control of the software that runs on those resources. In software as a service (SaaS), the service provider provides a software solution for the company to use, example being Google Docs. In platform as a service (PaaS), the service provider provides a platform for development. Everything as a service (XaaS) is an umbrella term for all of the service offering.

Wikipedia

Infrastructure as a service (IaaS) is a cloud computing service model by means of which computing resources are supplied by a cloud services provider. The IaaS vendor provides the storage, network, servers, and virtualization (which mostly refers, in this case, to emulating computer hardware). This service enables users to free themselves from maintaining an on-premises data center. The IaaS provider is hosting these resources in either the public cloud (meaning users share the same hardware, storage, and network devices with other users), the private cloud (meaning users do not share these resources), or the hybrid cloud (combination of both).It provides the customer with high-level APIs used to dereference various low-level details of underlying network infrastructure like backup, data partitioning, scaling, security, physical computing resources, etc. A hypervisor, such as Xen, Oracle VirtualBox, Oracle VM, KVM, VMware ESX/ESXi, or Hyper-V runs the virtual machines as guests. Pools of hypervisors within the cloud operational system can support large numbers of virtual machines as well as the ability to scale services up and down according to customers' varying requirements.