There are 3 types of cloud service models

IaaS (Infrastructure as a service)

PaaS(Platform as a service )

SaaS(Software as a service)

Source: Azure webservices

Lets know some Baiscs of this 3 models

Iaas(Infrastructure as a service): On this service, you get most of the controls of your cloud but you avoid the expense of buying and operating your own physical servers and other data centre infrastructure which are costly and complicated. But you get to manage the operating system, storage and networking attributes.

Uses of IaaS: When you want to run multiple application on a single vertual machine, testing your application on different OS, more control over your website.

PaaS(Platform as a service ): It is for simple things like website hosting. Here you do not need to worry about software licence, OS, storage and networking attributes. Instead, you manage the website or application you develop, so you are getting less control from IaaS.

Uses of PaaS:Web hosting,testing, building and updating.

SaaS(Software as a service):When you want the least management responsibility. Basically microsoft products like office 365, word etc use it(subscription based services).

Uses of SaaS: Hosting webservices like office 365.