What are main components in Docker architecture ?

Docker proposes a Client/Server architecture

• ensures OS agnostic approach (Run on any OS)
• Containers have all the libs, network info, execution environment needed for application

3 Main components in architecture
Host contains :

• Containers (N) and
• 1 Docker deamon (invisible thread which controls life cycle of containers) 

Client accesses to Docker deamon

 
DOCKER CLIENT
Set of binary libs and commands which runs at client side and communicates with Docker Deamon.
Example commands
  docker build :  Build a container from scratch
  docker pull : Fetch docker image from registry
  docker run : Run a docker image within docker container

DOCKER HOST
It is underlying OS which runs on top of Kernel services. It contains :

• Docker deamon
• Containers
• Images (Local repo/Cache/downloaded images)

DOCKER DEAMON

• Runs on Host machine 
• does all the heavy lifting needed by containers
• Client accesses / talks to Docker deamon, not directly to containers
• User doesn't access to Deamon directly

REGISTRY
Central repository of docker images / templates that can be pulled into a Docker container.
It could be public (on internet) or private repositories.

Huge public repo is Docker hub (hub.docker.com) which also provides auth, tools, web hooks.
You should create account and login before access the repo using command : docker login