Just to help some viewers who may have become confused, both Breplum and Frodo are correct. In states such as California, a true air gap is required. The state where Frodo lives, it is not required. I live in California, have a beautiful chunk of granite installed in my kitchen, and refuse to have that unsightly air gap exposed next to my sink, so I chose a high loop discharge. I also had 4 local inspectors survey my home when purchasing it, and not one inspector mentioned the air gap missing.
Here is an article I found on the web, which may help put this debate to bed, once and for all...
Though a high loop is a classic and effective method of backflow prevention, it is not as safe as an air gap. A high loop cannot assure back siphonage prevention. When the water pressure on the supply side drops significantly, water flow can reverse and the dishwasher drain line can suction dirty water and bacteria into the appliance. For example, if you have a double sink in your kitchen and they are draining simultaneously, this could cause a pressure differential that could lead the dishwasher to siphon water back into it through the drain line. There’s also the risk of the high loop becoming loose and sagging or poor installation rendering it ineffectual. However, high loops are popular, inexpensive to install and have proven to be reliable backflow prevention methods. If you opt not to install a dishwasher air gap, a high loop is the best alternative.