Conventional ovens use a heating element (powered by gas or electricity) to produce heat. This heat is slowly dispersed throughout the oven. Convection ovens kick things a notch higher by using a fan to speed up hot air circulation in the oven.
Air fryers use a technology that is somewhat similar to this but not quite the same. In an air fryer, rapid air technology is used to produce heat instead of an element. This ensures that the unit heats up faster than a regular oven. The fact that air fryers are often smaller in size compared to ovens also aids the quicker circulation of heat and limits the possibility of hot spots. The result is crispier and more evenly cooked food.
Many people also consider air frying a much healthier alternative. Because it heats up quickly and is hot enough, you don't need to add any oil to your food to crisp it up. This is hard to replicate (or even impossible) with a regular oven. Using little or no oil means the dish is generally healthier.