Containers are running instances of images, with isolated filesystems, networks, and environment. Start a container with docker run; it boots instantly and includes all dependencies. Containers are ephemeral by default—data and changes disappear when they stop, unless you use volumes.

$ docker run -it ubuntu      # run interactively (exit to stop)
$ docker run -d --name myapp nginx # run in background
$ docker ps                  # list running containers
$ docker ps -a               # list all containers, including stopped
$ docker stop <container>    # stop a container gracefully
$ docker rm <container>      # delete a stopped container

Containers that exit remain on disk until you delete them. Use docker logs to see output, docker exec to run commands inside a running container, and docker restart to start a stopped one. Each container is isolated—changes inside don't affect other containers or the host.