The "it" flag is used in a RUN or EXEC command, which is a combination of two individual flags i and t.

i stands for interactive, which connects the user terminal to the STDIN of the container. The t flag refers to pseudo-tty which pretty prints the STDOUT channel of the container.

Example:

> docker run -it ubuntu bash

The above command runs an ubuntu container and connects the bash command output to the user terminal, which results in the ability for user to send and receive I/O onto the container.

START command is used to start an already created container. On the other hand, a RUN command is used to CREATE and START a non-existing container from a specified image.

So basically a docker RUN command does both CREATE and START commands combined.

> docker run [options] <image_name> [initial_command]

To start an already created container, we use the below command:

> docker start [options] <container_id> [<container_id> ..]

Where container_id is the unique identifier referring to a created container. This is the value returned when we run a docker create command.

A container for a specified image can be created in docker by the below command:

> docker create [options] <image_name> <initial_command>

This command takes an existing docker image name as a parameter and returns a container_id for the created container. This container_id is used for working with the created container

An image is a trimmed down version of operating system along with a specific set of resources pre-installed, which is downloaded from the docker repository and setup in the client machine (our machine) for use. In Docker, an image is used as a base in building a container which contains all the required dependencies along with the environment for the application to run without any hosting issues.

A container is an encapsulated environment which allows developer to pack all the necessary libraries and dependencies for running an application without any issues and deploy the entire container as an instance.