The Block of Fire can be made by putting 9 Fire in a Crafting Table. Any mob that touches the block will be set on fire, unless they have a Potion of Fire Resistance. TNT will explode when placed within 10 blocks.
Blocks of Fire can be used for cooking. They can not be placed in a Furnace. To cook with Blocks of Fire, you must right-click on a Block of Fire with an uncooked food item in your hand.