A grill is a piece of cooking equipment where the cooking surface consists of an open rack or grate with a heat source underneath. Depending on the type of grill, the heat source can be an open flame (either gas or charcoal) or electric.
Food is cooked directly on the rack or grate of the grill. Thus, the best kinds of foods for cooking on a grill tend to be meats and poultry, although firm fish and seafood and also vegetables can be cooked on the grill.
The word grill can also be used as a verb to indicate cooking on a grill. Sometimes the word grill can be used interchangeably with the word barbecue. But technically, barbecuing is a form of grilling over wood or charcoal.
One of the characteristics of food cooked on a grill are the grill marks from the grate or rack. This effect can be obtained by using a specially made pan called a grill pan. A grill pan has raised ridges that can produce grill marks, but cooking on a grill pan isn't really grilling.