IaaS (Infra as a Service)
IaaS provides the infrastructure such as virtual machines and other resources like virtual-machine disk image library, block and file-based storage, firewalls, load balancers, IP addresses, virtual local area networks etc. Infrastructure as service or IaaS is the basic layer in cloud computing model.
Common examples: DigitalOcean, Linode, Rackspace, Amazon Web Services (AWS), Cisco Metapod, Microsoft Azure, Google Compute Engine (GCE) are some popular examples of Iaas.
PaaS (Platform as a Service)
PaaS or platform as a service model provides you computing platforms which typically includes an operating system, programming language execution environment, database, web server. technically It is a layer on top of IaaS as the second thing you demand after Infrastructure is a platform.
Common examples: AWS Elastic Beanstalk, Windows Azure, Heroku, Force.com, Google App Engine, Apache Stratos
SaaS (Software as a Service)
In a SaaS, you are provided access to application services installed at a server. You don’t have to worry about installation, maintenance or coding of that software. You can access and operate the software with just your browser. You don’t have to download or install any kind of setup or OS, the software is just available for you to access and operate. The software maintenance or setup or help will be provided by SaaS provider company and you will only have to pay for your usage.
Common examples: Google Apps, Microsoft office365, Google docs, Gmail, WHMCS billing software