What is the difference between heartburn and acid reflux?

In response to a question on healthshare.com.au

Heartburn is a symptom - something you experience - the burning feeling in the chest or neck that can occur when acid flows up from the stomach into the oesophagus.

Acid reflux is the physical process of fluid (mostly hydrochloric acid) from the stomach flowing back up into the oesophagus. This can cause various symptoms - it can cause heartburn; regurgitation (the feeling of fluid flowing up into the throat); cough in the mornings, or not being able to clear the throat in the mornings.