from Wikipedia: https://en.wikipedia.org/wiki/Platform_as_a_service#Types

PaaS is derived from software as a service (SaaS), and is situated in cloud computing between SaaS and infrastructure as a service (IaaS). SaaS is software hosted in the cloud so that it does not take up hard drive space on the computers of users. IaaS provides virtual hardware from a provider with adjustable scalability. With IaaS, the user still has to manage the server, whereas with PaaS the server management is done by the provider.

A private PaaS can typically be downloaded and installed either in a company’s on-premises data center, or in a public cloud. Once the software is installed on one or more machines, the private PaaS arranges the application and database components into a single hosting platform.

So, Youtube: Saas . WPEngine: PaaS, and AWS: IaaS

Those are the basics – head over to the link if you want to read about mPaaS, CPaaS, Open PaaS, NaaS, and MBaaS.