Docker is a platform for developing, shipping and running applications inside a containers. Docker images are those templates which can be used by you to run applications.
You can use below link for installing docker on ubuntu system.
Without Service or Raw Image: Raw images are basically base images that servers as foundation for building other docker images. Base images have only core operating system and minimal software installed to run a application. It can ubuntu, Redhat, CentOS etc.
With Service or Serice Image: This is basically raw images installed with some functionality, application etc. You can use them for performing required tasks.