There are 3 types of Service models in Clouds.
Infrastructure as a Service
Platform as a Service
Software as a Service
General Diagram of these models to understand what cloud provider manages and what client manages.
IaaS (Infrastructure as a service)
IaaS is on-demand access to cloud-hosted computing infrastructure - servers, storage capacity, and networking resources - that customers can provision, configure and use in much the same way as they use on-premises hardware. The difference is that the cloud service provider manages and maintains the hardware and computing resources in its own data centers. IaaS customers use the hardware via an internet connection and pay for that use on a subscription or pay-as-you-go basis.
PaaS (Platform as a Service)
PaaS provides a cloud-based platform for developing, running, and managing applications. The cloud services provider hosts manages and maintains all the hardware and software included in the platform - servers (for development, testing, and deployment), operating system (OS) software, storage, networking, databases, middleware, runtimes, frameworks, development tools - as well as related services for security, operating system and software upgrades, backups and more.
SaaS (Software as a Service)
SaaS (sometimes called cloud application services) is cloud-hosted, ready-to-use application software. Users pay a monthly or annual fee to use a complete application from within a web browser, desktop client, or mobile app. The application and all of the infrastructure required to deliver it - servers, storage, networking, middleware, application software, and data storage - are hosted and managed by the SaaS vendor