IaaS, PaaS, and SaaS are the three fundamental cloud service models.

Infrastructure-as-a-Service (IAAS)

• IaaS allows the cloud provider to freely locate the infrastructure over the Internet in a cost-effective manner.
• IaaS allows the customer to access computing resources through administrative access to virtual machines.
• IaaS provides access to fundamental resources such as physical machines, virtual machines, virtual storage, bandwidth, IP addresses, monitoring services, firewalls, etc. to the customers on rent.
• The payment is based upon the amount of time the customer retains a resource.
• Also with administrative access to virtual machines, the customer can run any software, even an operating system.
• IAAS is generally used by network architects.
• Examples: Amazon Web Services (AWS), Rackspace, Cisco Metapod, Microsoft Azure, Google Compute Engine (GCE)

Platform-as-a-Service (PAAS)

• PAAS gives access to run time environment to deployment and development tools for application.
• PaaS has a feature of point-and-click tools that enables non-developers to create web applications.
• In this customers need not bother about the administration because it is the responsibility of the cloud service provider.
• Customer need not purchase expensive hardware, servers, power, and data storage.
• It is very easy to scale the resources up or down automatically, based on their demand.
• It is the responsibility of the cloud service provider to maintain software versions and patch installations.
• PAAS is generally used by developers.
• Examples: AWS Elastic Beanstalk, Windows Azure, Heroku, Force.com, Google App Engine, Apache Stratos, OpenShift, and Magento Commerce

Software-as-a-Service (SaaS)

• SaaS model allows to provide software applications as a service to the end-users.
• It refers to software that is deployed on a hosting service and is accessible via the Internet.
• The SaaS application deployment requires a little or no client-side software installation.
• Therefore, there is no requirement for complex software packages on the client-side, little or no risk of configuration at the client-side, and low distribution cost.
• The customer can have a single license for multiple computers running at different locations which reduces the licensing cost.
• Also, there is no requirement for license servers because the software runs in the provider's infrastructure.
• The cloud provider stores data centrally. However, cloud service providers may store data in a decentralized manner for the sake of redundancy and reliability.
• All platform responsibilities such as backups, system maintenance, security, hardware refresh, power management, etc. are performed by the cloud service provider. The customer does not need to bother about them.
• Multitenant solutions allow multiple users to share a single instance of different resources in virtual isolation.
• Customers can customize their application without affecting the core functionality.
• SaaS provides us Application Programming Interface (API), which allows the developer to develop a customized application.
• SAAS is generally used by the end-user.
• Examples: Google Workspace, Dropbox, Salesforce, Cisco WebEx, Concur, GoToMeeting, Gmail, Slack, and Microsoft Office 365