Process types and the Procfile
Procfiles were introduced by Heroku as a mechanism for declaring what commands are run by your application. See this link for more details.
Process types are declared in a specific file named Procfile. It comes from the Heroku way of doing things, and is a simple text-based file spercifying a mapping between process names, and how those process are launched.
For instance, most web applications have at least a web process, which is usually a server daemon being able to serve HTTP requests for the application. Applications may also have a worker process, which is used to run background job processors, such as Sidekiq or Resque in the Ruby world.
Process names can be arbitrary, although it is expected that the web process specifies the main process of your application.
Launching processes
Any process declared in the Procfile can be launched using the dedicated command line tool that comes with each package. For instance, assuming you have declared the following Procfile:
web: bundle exec rails s -p $PORT
worker: bundle exec sidekiq
You would be able to launch any of the web or worker processes in two way:
1) As a foreground process
sudo my-app run web
To quit the process, you would hit CTRL-C.
2) As a daemonized process
sudo my-app scale web=1
Using the scale command will register the process with your init system.
As such, to manage the process, you would then use the service utility from your system:
sudo service my-app-web stop|start|restart|status