SaaS == Software As A Service. Basically cloud hosted software, web applications, etc. Usually accompanied by a recurring fee.
PaaS == Platform As A Service. Think providers like Digital Ocean, Microsoft Azure, Amazon Web Services, who offer virtual servers, database hosting, etc.
You may well use docker or kubernetes together with your chosen PaaS provider, but not necessarily. Docker and Kubernetes are both pieces of software for "containerising" applications. This essentially makes it easier to deploy your application to different platforms as you can standardise the parts of the environment that are relevant to your application. That's a topic for a whole different post.
BaaS == Backend As A Service. I admit this isn't a term I'd heard before. BaaS providers offer services that let you easily store data, files, handle push notifications, etc. Assuming you don't have any needs that aren't served by their API, it means you can focus entirely on your front-end code, integrate with their API, and they handle the majority of the backside of your application.
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