The past looked like this: A User logs into a Computer, launches a Program, and interacts with it.
The future looks like this: The Computer on your desk runs a Program (in the background) that collaborates with a Program running on the Computer in your pocket and another Program running on a Computer in the Cloud, operating on your behalf without the need to interact.
In the past, a Program and an Application were the same thing. More and more, the Applications of today and tomorrow are made up of multiple Programs running on multiple Computers but cooperating with each other to achieve some utility for You (formerly the User).
The web development community has lately been very excited about single-threaded event-driven servers like Node.js. These processes are very good at maintaining a large number of connections, each of which requires only a small amount of work. (These servers are not very good at the inverse case, a small number of clients asking for very hard work to be done. For that, you want a different model.)
This paradigm of a large number of connections and small amounts of work fits neatly into the world where large numbers of processes collaborate to create a useful result. Each process does a relatively small amount of work, but the value emerges from the coordination of the processes through their communication.
Example: There is a process on your phone that displays emails. There is a process on the mail server that sends the messages to your phone. There is a process that examines messages as they arrive at the server to filter out junk mail. There is another process that examines the messages to rank them by importance and places some in your Priority Inbox. These processes are constantly running, on multiple servers, operating on your behalf in the background.
Years ago, Tim O’Reilly was writing about software above the level of a single device as part of his Web 2.0 concept. Tim’s classic example is the iPod-iTunes-iStore triumvirate. You have servers on the Internet, a desktop or laptop computer, and a small handheld device all coordinating your data for you.
As more devices have computers embedded into them, there are more opportunities for cross-device applications. And as more such applications emerge, users will expect applications to coordinate across devices like this. If you are designing a new application today, you’d better be thinking about it as a distributed system of cooperating processes.