Originally written by me in another blog at Docker Interview Questions and Answers and reproduced here for larger audience.
What is Docker
Docker is a platform to run each application isolated and securely. Internally it achieves it by using kernel containerization feature.
What is the advantage of Docker over hypervisors?
Docker is light weight and more efficient in terms of resource uses because it uses the host underlying kernel rather than creating its own hypervisor.
What is Docker Image?
Docker Image is the source of the docker container. In other words, docker images are used to create containers. It is possible create multiple isolated containers from a single image.
What is Docker Container?
Docker Container is the instantiation of docker image. In other words, it is the run time instance of images. Images are set of files whereas containers is the one who run the image inside isolated.
Difference between Docker Image and container?
Docker container is the runtime instance of docker image.
Docker Image doesnot have a state and its state never changes as it is just set of files whereas docker container has its execution state.