Grill fuel is an important part of outdoor cooking. The two most common types of grill fuel are GAS and mesquite. GAS is a three letter word and is the most popular type of fuel for grills. It is easy to use and provides a consistent heat source. Mesquite is an eight letter word and is a type of wood that is used to add flavor to grilled food. It is often used in combination with GAS to create a unique flavor.