A single local machine can have multiple programms running simultaneously on it. Each process or application will share the same underlying physical resources of the local machine. These resources include, processing power, storage and memory.
The above process works fine, until multiple processess start competiting for the same resource. It can be when two different applications try to access the same file or try to run on same port. This can lead to conflict and one of the applications is bound to fail.
The intitutive solution would be to isolate these different application and run them in an isolated environment. This environment is provided by a Virtual Machine(VM). A virtual machine (VM) is a virtual environment that functions as a virtual computer system with its own CPU, memory, network interface, and storage, created on a physical hardware system
This VM will run on the local machine (or remote machine) and will consume the physical resources (RAM,memory and processing power). Each VM is contains its own operating system complete with applications required to run a particular application. Most commonly used VM images are linux based. A VM will emulate the functionality of a separate physical computer.
Performance
Even though the machines are virtualized in a virtual machine, it still relies on the resources from the host machine. A computer needs to be powerful enough to run several virtual machines on a single host computer. If its power is not sufficient enough, it will cause stability issues in the performance.
Speed
Since virtual machine access the physical machine indirectly, the time to boot
up and process operations are slower. It might take several minutes to boot up a
virtual machine, since effectively its is starting a new machine and has to
perform the OS boot operations. 
Docker is an open platform for developing, shipping, and running applications. Docker enables you to separate your applications from your infrastructure so you can deliver software quickly.
Docker provides the ability to package and run an application in a loosely isolated environment called a container. The isolation and security allows you to run many containers simultaneously on a given host. Containers are lightweight and contain everything needed to run the application, so you do not need to rely on what is currently installed on the host.
Consistent Environment
Consistency is a key benefit of Docker wherein developers run an application in a consistent environment right from design and development to production and maintenance. As such, the application behaves the same way in different environments, eliminating production issues.
Speed and Agility
Speed and agility is another key benefit of Docker. It allows you to instantly create containers for every process and deploy them in seconds. As you don’t have to boot the OS, the process is done lightning fast. Moreover, you can instantly create, destroy, stop or start a container with ease. By simply creating a configuration file using YAML, you can automate deployment and scale the infrastructure at ease.
Docker creates a level of abstraction on top of the local machine and allows launching application in self sufficient and self contained enviroments. Before we can launch sucn an enviroment its important to understand how an application get containerized.
Your entire application is bundled into an image. This image acts as a single source of reference which can be used to create copies of application (containers). Each container will have the same behavior since it is generated using the same build image.
A Docker image can be build by using text configuration written in a Dockerfile. Using the docker build command one can create a docker image
docker build ./path-to-Dockerfile image-name
This command generates an image as per specifications in the Dockerfile. A new container can be started simply using the image-name given in the previous command. Please know that this build is a one time step and a new image will only be build when either the code changes or when some configuration changes in the Dockerfile.
A new container can be created from an image using the docker run command.
docker run image-name
The run command can be executed with differnt options, some of them are presented in the image below. Complete list can be found in the official Docker documentation.
