Infrastructure as a Service (IaaS) means you’re buying access to raw computing hardware over the Net, such as servers or storage. Since you buy what you need and pay-as-you-go, this is often referred to as utility computing. Ordinary web hosting is a simple example of IaaS: you pay a monthly subscription or a per- gigabyte fee to have a hosting company serve up files for your website from their servers.
Software as a Service (SaaS) means you use a complete application running on someone else’s system. Adobe Creative Cloud Suite or Hosted Exchange are perhaps the best-known examples.
Platform as a Service (PaaS) means you develop applications using Web-based tools so they run on systems software and hardware provided by another company. So, for example, you might develop your own eCommerce website but have the whole thing, including the shopping cart, checkout, and payment mechanism running on a merchant’s server.
Desktop-as-a-service (DaaS) is a form of virtual desktop infrastructure (VDI) in which the VDI is outsourced and handled by a third party. Also called hosted desktop services, desktop-as-a-service is frequently delivered as a cloud service along with the apps needed for use on the virtual desktop.