IaaS : Infrastructure as a solution is a concept by which hardware components such as compute server, storage and networking components are provided. In this case, customer/client decides what OS should be installed. IaaS is considered at bare metal.
PaaS: When an OS is installed in bare metal components it becomes platform as a service and user can launch Virtual machine. And he/she can install applications within the VM as per requirement.
SaaS : In SaaS, services are provided to customer/client such as Office365 through cloud platform. In this special case user is taking a particular service. User does not bother in which type of platform services are running.