I am excited to write this article today, before I go into the topic, I'd like to buttress these terms for a bit, just because we can't really discuss app services or Azure Fundamentals without highlighting and grasping these key terms:
IaaS, PaaS and SaaS
I'd start off with this analogy of the terms, just to share an insight and help the concepts stick better
In the App Services space, Your App is a Car and let's assume there are 3 different Scenarios for you to access this car
Purchasing a car - Purchasing a car comes with responsibilities such as: outright/installment payment, securing a driver's license, car documentation and registration, fuel, periodic maintenance, you will need an insurance plan especially if the car is brand new, and many other responsibilities that you have to take on solely, although you are not the manufacturer of the car, you are only a customer who made a "happy" purchase.
Recall that a very common example of IaaS (Infrastructure as a Service) is a Virtual Machine and the scenario described above "corresponds with - IAAS where you purchase, maintain and secure your own virtual machine. You own the Virtual Machine but it means you have full responsibility whenever something goes wrong, the cloud service provider is only responsible for the physical security of the datacenter and other hardware components , just like the manufacturer of the car.
Hiring a car: When you hire a car you can choose what car model you get, how fast you want drive it, you get to choose the kind of car you want and the specifications, you can decide to go back and upgrade your car- you don't have to worry about all the infrastructure and the maintenance of it. When you have a web app running on PaaS, you do not worry about the VM and the maintenance of it, all you just have to do is bring your code, host it on the platform, make changes as at when necessary and let the cloud service provider handle the maintenance and be responsible for it.
Taking a Cab: You order a Taxi, Uber or Bolt and all you care about is sitting in the car and getting to your destination. You are not responsible for maintenance, driving, you don't exactly choose the model of the car or the driver, no need to have a driver's license to order a cab, you basically just order it and enjoy the ride. This scenario corresponds with SAAS (Software as a Service) - the product is pre-built for you, think of your everyday applications: Team, gmail, quora, Uber, facebook etc. You simply download the app and access the features therein, no responsibility for upgrades or the latest versions, you are the end user of the product.
I hope I have been able to give you a better understanding of these concepts using these analogies, it helped me as a beginner in my app service class.