'Dolphin,' I said. 'No,' my husband Barry insisted, 'a dolphin is a fish, the marine mammal is a porpoise!' In revising my book 'Marsh Mud and Mummichogs,' I found that I had carelessly referred to the small cetaceans that hunt and play along the Georgia coast as 'porpoises,' rather than the correct 'dolphins.' 'They are in fact bottle-nosed dolphins,' I replied as I changed the wording in the text. 'Porpoises are related to dolphins, but are in a separate family.' (This is the sort of thing couples who are both marine ecologists argue about.)
"Tursiops truncatus 01-cropped" by NASA - http://mediaarchive.ksc.nasa.gov/ detail.cfm?mediaid=21807. Licensed under Public Domain via Wikimedia Commons
Barry was right; there are two species of dolphinfish in tropical waters. Hawaiians call these fish 'mahi-mahi', the label that is usually encountered on restaurant menus. Dolphinfish are delicious. It is confusing that a large family of marine mammals is also referred to as 'dolphins,' but there you have it. (The word 'dolphin' may have derived from the Greek 'delphys' meaning 'womb,' from the observation that these aquatic animals produce live young.)
'Porpoise' derives from a Medieval Latin term meaning 'pig-fish,' as these cetaceans were delicacies in Greek and Roman cuisines. There are only six species of porpoise worldwide, two of which have no dorsal fins.
Dolphins are much more abundant; there are 40 recognized species. The common bottle-nosed dolphin, Tursiops truncates, the only species found in Georgia estuaries, ranges all over the world's oceans and is the dolphin exhibited in marine aquaria (and in TV shows - remember 'Flipper'). When we lived on Sapelo Island, our family delighted in watching pods of dolphins cavorting in the waves just off the beach, or hunting schools of mullet in tidal rivers. They a have a clever method of catching prey. Snorting and splashing up waves in tight formation, the dolphins intentionally panic mullet into jumping onto the creek banks; then they lunge halfway out of the water to snatch up the stranded fish.
Georgia Outdoors has a great video about bottle-nosed dolphins, which delves into many aspects of the behavior of these intelligent animals, including their pack-hunting strategy.