What is Docker used for?

Docker is used to isolate and run applications in containers. A container holds everything the program needs to run: the environment, dependencies, packages, configs. Main tasks:

1. Isolate applications - so they don't conflict with each other and behave the same on any machine.
2. Simplify deployment - the application starts in any environment with one command, without manually configuring servers.
3. Standardize environments - development, testing, and production look the same.
4. Save resources - containers are lighter and faster than virtual machines.

Principle: build an image → run a container → get a predictable environment.