Comment by jimbokun

Sometimes.

Sometimes it is one or more programs writing to a queue or topic, and other programs reading from that topic.

Or programs writing to and others reading from a Unix pipe.

Or programs talking to each other using HTTP.

Or Erlang processes communicating concurrently on one machine or across a network.

Or different programs sharing one database.

Or many objects communicating by passing messages in a small talk program.

There are many ways to encapsulate programs and have them interact.