Answer Description
PaaS provide high level services like web and database servers without allowing access to the underlying operating system (Azure and Amazon web services are two examples of this). SaaS provides a software or application that is shared among all users (no dedicated server for your company, think Google Docs or Office 365). IaaS provides lower level services such as networking and a hypervisor, the OS would need to be installed and built up. (Azure, Amazon and many other providers exist for IaaS). iCloud is proprietary to Apple and is more of a filesharing/synchronization service.
Wikipedia
Platform as a service (PaaS) or application platform as a service (aPaaS) or platform-based service is a category of cloud computing services that allows customers to provision, instantiate, run, and manage a modular bundle comprising a computing platform and one or more applications, without the complexity of building and maintaining the infrastructure typically associated with developing and launching the application(s) and to allow developers to create, develop, and package such software bundles
Platform_as_a_service - Wikipedia, the free encyclopedia