General terms to understand Docker

• Container
• Docker Container
• Image
• Virtual Machine

Container (General Term)

Overview

A container is a standard unit of software that packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another.

• It relies on features built into the Linux kernel (primarily namespaces and cgroups) to create isolated environments
• Applications inside a container behave as if they are running on their own operating system, but in reality they share the host OS kernel.

• Key characteristics
  • Optimized to run a single process
  • Isolated filesystem, network, and environment variables from other containers
  • Lightweight and fast (much faster than VMs, often within seconds)
• To not be confused with docker
  • Before docker became popular, people were already using container technologies like LXC (Linux Containers) and Jails (on FreeBSD), but they were often complex to set up and manage.
  • All Docker containers are containers, but not all containers are Docker containers
    • Docker Container is just a type of container

Image

Overview

A static, executable package that defines how a container should run (the blueprint for containers).

• A read-only, static blueprint or template that contains everything needed to run an application inside a container
• An image typically contains:
  • Minimal OS components (e.g., Alpine, Ubuntu minimal)
  • Application executables (.jar, .py, .exe, etc.)
  • Dependency libraries, config files, environment variables
  • Instructions defined in a Dockerfile (COPY, RUN, CMD, etc.)

Virtual Machine

Image vs. Container vs. Virtual Machine

Aspect	Image (Docker)	Container (Docker)	Virtual Machine (VM)
Definition	Blueprint for containers	Running instance of an image	Emulated hardware running a full OS
Abstraction	Application package	Application runtime	Entire OS + kernel
OS Kernel	Uses host kernel	Uses host kernel	Separate guest OS kernel per VM
Startup Speed	N/A	Seconds	Minutes
Resource Usage	Very small (MBs)	Small (MBs–low GBs)	Heavy (GBs)
Isolation	Process-level isolation	Process-level isolation	Full system isolation (hardware-level)
Portability	High (same image runs anywhere)	High (container runs anywhere Docker runs)	Medium (depends on hypervisor compatibility)
Use Case	Build artifact	Run lightweight apps, microservices	Run full OS, legacy apps, strong isolation