Acronyms SaaS, PaaS and IaaS are heavily used in cloud computing industries and are the models for online services that a cloud offers.

IAAS stands for Infrastructure as a Service and provides the fundamental building blocks for cloud services. Without IaaS, both PaaS & SaaS can’t be developed and thus it happens to be the base level for online services.

It enables users to get access to their own infrastructure - computers, networking resources, storage. It's worth noting that these are typically virtual resources, but could be could be real, physical resources.

Amazon Web Services (AWS) and goDaddy are a few well-known examples of IaaS.

PAAS stands for Platform as a Service i.e. it provides a platform on which a software can be developed and deployed. As with most cloud services, PaaS is built on top of virtualisation technology. Businesses can requisition resources as they need them, scaling as demand grows, rather than investing in hardware with redundant resources.

It abstracts away the infrastructure (users don’t get to see the computers, loadbalancers, etc.) but rather provides a software development platform. It is possible to code and run an application on a PaaS and the system makes sure that the app has the necessary infrastructure to make it run and scale.

Google App Engine, Heroku and OpenShift are examples of IaaS.

SAAS stands for Software as a Service and is the top layer of cloud computing, it typically built on top of a Platform as a Service solution, whether that platform is publicly available or not, and provides software for end-users such as email, word processing, or a business CRM. Software as a Service is typically charged on a per-user and per-month basis, and companies have the flexibility to add or remove users at any time without addition costs beyond the monthly per-user fee.

Use of SaaS applications tends to reduce the cost of software ownership by removing the need for technical staff to manage install, manage, and upgrade software, as well as reduce the cost of licensing software. SaaS applications are usually provided on a subscription model.

For example, Ola is an SaaS.