A gas grill uses propane to light one or more rows of flame beneath the cooking surface. Some gas grills have briquettes that sit on a second grill over the flames, while others do not.
One of the benefits of a gas grill is convenience. Lighting the grill takes a lot less time than waiting for charcoal to catch fire and for the briquettes to burn down so that cooking can begin. The only step involved with lighting a gas grill is turning the gas on and hitting the igniter switch (or sliding a long match into the access hole, depending on the model). With a charcoal grill, one has to apply starter fluid to the briquettes and wait for them to ignite and burn down.
One benefit of a gas grill is the lack of a mess. At the end of a charcoal grilling session, there is a bunch of ash in the bottom of the grill, but with a gas grill, it is only necessary to rub the debris from the food off the grill with a brush and turn the gas off.
One disadvantage of a gas grill is the comparative lack of flavor. Charcoal briquettes do leave a distinctive flavor on the food during the cooking process, but it is possible to use spices and marinades to pre-flavor foods before putting them on a gas grill.