This assumes you have installed Docker on Linux and Docker is running.
A Docker image contains the code used to create a Docker container, such as creating a Nginx web server, or a mySQL server, or a home grown app, and the list goes on. In this way, an image is like a template used to create a container. An image is kind of like a virtual machine, but much more light weight, using significantly less storage a memory (containers are usually megabytes in size).
The docker images command will return the images you've downloaded.
Which will return something like this.
REPOSITORY TAG IMAGE ID CREATED SIZE foo latest 105b54dc64f1 32 hours ago 196.7 MB bar latest 95ga44bc5582 2 weeks ago 967 B
The docker info command can be used to return the number of images.
docker info . . . Images: 2