If you have Acrobat, you add form fields. In Acrobat 11, you'd select: Tools > Forms > Edit
When you do this it will ask if you want Acrobat to detect form fields for you. This is known as the Form Wizard and it will attempt to divine where it thinks form fields should go based on the content. It work fairly well sometimes, but you will often want to make refinements such as changing the types of fields, the field names, certain field properties, etc.
You decline this automatic field generation and create them all yourself using the various tools available in form editing mode.
More information is available in Acrobat's help and numerous tutorials at http://www.AcrobatUsers.com