Docker
In plain terms
A tool that packages apps into portable containers that run the same everywhere.
Going deeper
Docker bundles your code with its dependencies and config into an image, then runs it as an isolated container sharing the host OS kernel. This kills 'works on my machine' problems and makes deployment consistent and fast.
Under the hood
Images are layered and cached for efficiency; containers share the host kernel (not a hypervisor), making them fast but less isolated than VMs. Dockerfiles define builds.
Examples
docker build/run, Dockerfile, Docker Compose, multi-stage builds, image registries.
Think of it like...
Like shipping goods in a standardized cargo container: whatever's inside, every ship, crane, and truck handles it the same way.