IaaS or Infrastructure as a Service is a cloud computing service that provides access to computing resources in a virtualized environment. In IaaS the computing resource is provided specifically of the computer infrastructure. It includes the offering of virtual space, network connections, bandwidth, IP address and load balancers. Platform as a Service (PaaS) is a category of cloud computing that provides a platform and environment which allow developers to build applications and services over the internet. PaaS are hosted in the cloud and can be accessed by simply accessing the web browser. SaaS or Software as a Srvice is a cloud service where consumers are able to access software applications over the internet. Some examples of SaaS are Twitter, Google and Facebook.
To give you a visual idea, here’s an infographic: