It's really an set of tools for lxc. Create "images", boot them, and have a sandboxed execute environment for some service(s). There are repositories with services pre installed. You can get a service running with a command like:

docker run -h myhost -t repo/redis-image redis-server

That creates a container based on the repo/redis-image image, and runs the command "redis-server" once it boots (less than a second on my machine).