Most people do not realize that there is a difference between these two actions.
Vomiting is an active process.
There is gagging, heaving, and retching as the body actively expels stomach contents. Regurgitation is passive. For whatever reason, food is swallowed from the mouth but never really goes anywhere beyond that point. Food sits in the esophagus until it simply falls back out the mouth at some point. In the dog, megaesophagus is the most common cause of regurgitation.