In infrastructure as a service, you get the raw hardware from your cloud provider as a service i.e you get a server which you can configure with your own will.
For Example: Azure VM, Amazon EC2
Platform as a Service, gives you a platform to publish without giving the access to the underlying software or OS.
For example: Web Apps, Mobile Apps in Azure.
You get software as a service in Azure, i.e no infrastructure, no platform, simple software that you can use without purchasing it.
For example: when you launch a VM on Azure, you are not buying the OS, you are basically renting it for the time you will be running that instance.