Inside a gas grill, first there are the burners to create the heat. Above the burners, there needs to be a system to disperse the heat such as flavorizer bars, ceramic briquettes or lava rocks. Then come the cooking grates to place the food.
Better quality grills have two or more separate burners, not just control knobs, which allow for a greater control of heat. Inexpensive gas grills will have one H-shaped burner or bar, some with one or two control knobs. Keep in mind that grills with only one burner don't allow you to control the heat very well and will result in hot and cold spots on the cooking surface. While cooking, the juices from food drip down and sit next to the heat source until reaching a "flash point" and burn off. The best systems flash the droppings quickly and eliminate the chance for a flare-up while creating flavorful smoke.
Another feature to be aware of when looking for a gas grill are the BTU's (British Thermal Units). BTU's indicate the amount of gas a grill can burn - not its cooking power. A grill with too many BTU's can result in damage to the cooking elements and the grill itself. However, grills with fewer BTU's cook the food more efficiently. Just remember, a large grill with a large cooking surface capacity will need a higher number of BTU's.