The direct cooking method is when food is placed directly over the heat. This method is ideal for dishes that take less than 15 minutes to cook or that need a nice sear, such as steaks and burgers.
The indirect cooking method is where food is not placed directly over the heat.The food will cook "indirectly" through the heat circulating inside the barbecue. The indirect cooking method is ideal for foods that take longer than 30 minutes to cook, such as a roast.
The 50/50 cooking method uses a combination of direct and indirect cooking; this method can be used for food that takes between 15-30 minutes such as thicker cuts of steak & sausages.
See here for more information on cooking methods.