A container is a binary blob that contains everything your application needs to run. All files, dependencies, other applications etc.

Unlike a VM which abstracts the whole OS a container abstracts only your app.

It uses path manipulation and namespaces to isolate your application so it can't access anything outside of itself.

So essentially you have one copy of an OS rather than running multiple OS's.

It uses way less resources than a VM.

As everything is contained in the image if it works on your machine it should work the same on any. Obviously networking and things like that can break it.

Think of a container like a self contained box that can be configured to contain everything a program may need to run.

You can give the box to someone else, and they can use it on their computer without any issues.

So I could build a container that contains my program that hosts cat pictures and give it to you. As long as you have docker installed you can run a command "docker run container X" and it'll run.