Actually, is not very complicated. The main problem is that people, from both sides of the fence, eg customers and the aws/goog/ms people are conflating things, either deliberately or by simply lack of understanding.

I'm going to simplify some of the things, please don't get your pitchfork out if I gloss over some aspects.

Consider a web app, you most likely store your data in database, have some code for the business logic, say php and a bunch of front end stuff, html and js. You set up a web server, say apache, with and interpreter for your business logic code, say mod_php and a db server, mysql, on the same physical server (or virtual server). Your www.something.com is up and running.

Fast forward some time and your business grows. You notice that access to your website is very slow, since there are so many people trying to use your website. You move the db on another physical server to free more resources for the business logic. This doesn't last for long and your website is slow again. You also notice that your db server is not doing that much, yet the app server is being hammered.

You set up another app server, call it ww2.something.com, rename your original one to ww1.something.com and connect both of them to your db server. You also set up a third server, way smaller than the app servers, that on requests to www.something.com, based on some heuristic, will http redirect the user to either ww1. or ww2. [glossing over the db details here]. You see this works and you keep adding wwxxx.something.com. One day, on 20th of December, after you finished deploying ww1337.something.com, you realize you have another problem. How on earth are you going to deploy the brand new version of your webapp, that your team worked hard to finish before Christmas, in time for 25th? And there was a new CVE published, apache ver 1234 is vulnerable to the Grinch exploit. Not to mention that you expect a huge spike in traffic from 25th to 10th of January. So you need to add more servers. You also need to pay for these extra servers all the way to 25th of January and you start to wonder if this is worth it. And ww1256 has a hardware failure. Or was it ws1265?

Some time after, you had a chat with the CEO of this hosting company, he wants you to stop using bare metal and instead, move your stuff on virtual machines and they will make sure that all our vms will run, with an uptime of 99.99%, on their cluster. They also give you some tooling to build virtual machine images, thus a new deploy takes hours now, instead of days. No more worries about hardware failures [glossing over], if you find vulnerabilities, you can react on hours.

With the extra money made by being able to quickly deploy new code, hence develop new features, you hire more people and grow your business even more. One of the new people pokes around for some time and comes up with a list of things she thinks can get better, like your web app has powerusers and the http redirect load balancing makes everybody that happens to end up sharing a server with such an user, terrible miserrable, you should load balance based on request, not session. Most of the features are fairly light, resources wise, just a handfull are heavier. You should offload these requests to another set of bifier, more expensive vms, so while the main app servers wait for this forwarded request to happen, they can asnwer a bunch of the ligher requests. You agree and add the final piece in the puzzle, the application load balancer and move some of the features to a different set of resources.

Hope this clarifies what some of these pieces do. Next, a brief list of real life equivalents for the above:

1) vms: any technology that allows you to have a piece of code run in isolation. VMWare images, Virtual Private Servers, docker containers, this is NOT a new technology.

2) cluster: any group of CPU/Memory/Storage you can use to run vms. AWS Fargate is one, the entire EC2 is another one. An ECS Cluster of EC2 instances is yet another one.

3) tooling: whatever you use to create vms and instruct the cluster to run your vms. ECS is the pure AWS solution. Docker compose is another one. Kubernetes is another one. EKS is the aws offering for managed kubernetes. GKE is the google version.

/edit: GKS -> GKE