Docker is a developer tool which help us to run applications in an isolated environment, where you can start a service, app,local host, MySQL, SQL server, mongodb etc and they can can communicate each.
Images are used to create container which are used to run apps and services. You can find image of Redis, Mongo, MySQL, Ubuntu and everything needed for a develop and test app @ https://hub.docker.com/
Find the image you want and copy the docker pull code.
Open the command prompt. Before start downloading I will show you how you can view a list of Images you had
This will list all the docker images in your computer. Lets download nginx web server
docker pull nginx docker images