Machine names are all over the place in Drupal 7. When you create a content type or field it will appear next to the label. The concept of a machine name is simple. It's an alphanumeric value which shouldn't change once created and it must be unique.

A custom machine name can be implemented in two ways: as a field or form element. If you're creating a form you can implement one using machine_name form element. On content types, you can use the Machine name field.

In this tutorial, you'll learn how to setup and use the machine name field on an article content type.