What is Docker

Docker is the most popular containerization platform. It packages an application and all its dependencies (code, runtime, system tools, libraries) into a single lightweight, portable unit, enabling "build once, run anywhere".

Why containers

The classic pain is "it works on my machine": dev, test, and prod environments drift, causing dependency conflicts and version mismatches. Containers solve this with isolation + standardization:

• Isolation: each container has its own process space, filesystem, and network — no interference.
• Standardization: identical behavior on a laptop, a server, or a dev board.

Containers vs Virtual Machines

Aspect	Virtual Machine (VM)	Container
Virtualization layer	Hardware + full Guest OS	OS process level
Startup speed	Seconds ~ minutes	Milliseconds ~ seconds
Resource cost	High (full OS per VM)	Low (shares host kernel)
Isolation strength	Strong (kernel-level)	Medium (namespace-level)

💡 Containers share the host kernel, so they are lighter than VMs — but you cannot run a container whose kernel differs from the host (e.g. a Windows container on a Linux host needs extra setup).

Core components

Docker uses a client-server architecture:

• Docker client (CLI): the docker commands you run.
• Docker daemon: the background service that builds, runs, and distributes containers.
• Docker registry: the hub that stores images; Docker Hub by default.

tip

Typical workflow

Write Dockerfile → docker build image → docker push to registry
                                      ↓
                          docker pull / docker run container

Next, read Core Concepts to learn how images, containers, volumes, and networks relate.