Relationship between multiple containers is a decent way to understand what a pod in the world of k8s is. As you probably know containers provide operating-system-level virtualization. In Linux this virtualization is achieved via namespaces. By default, all containers within a pod share network, and ipc namespaces. In v1.13 you can also set the shareProcessNamespace to share the process namespace between containers in the same pod. See here for more info on this.

There is a good article here with some examples for communication between containers of the same pod leveraging the shared namespaces (and also volumes).

Also see this proposal by @brandandburns to make a pod a first class object in the docker API.