1. IaaS or Infrastructure as a Service: IaaS is a self service model for monitoring and controlling local data centers, storage, network and networking devices. Instead of buying infrastructure like data centers or hardware for you business, you can avail services from IaaS providers on subscription basis.
Example of IaaS providers are AWS, Microsoft Azure, etc.
2. PaaS or Platform as a Service: This service is used as a medium for developing applications without having to install the software on your desktop locally. It makes the development, testing, deployment and maintenance of applications easy and quick.
One example of PaaS is Force.com by Salesforce.
3. SaaS or Software as a Service: It has the major users of the cloud market. SaaS uses web to deliver applications hosted and managed by third party vendor and whose interface is accessed on the client side. Because of SaaS, applications can be accessed from anywhere and on any device connected to the web.
SaaS examples are Salesforce, Cisco WebEx, Google Apps, etc.
You can read this article for more undersatanding: https://apprenda.com/library/paas/iaas-paas-saas-explained-compared/