Wort chillers are used to cool the hot wort down to room temperature before you pitch your yeast. Immersion chillers
work by first completely submerging the chiller in your kettle and then by flowing cold water through the coils (Make sure to stir your beer as it's being chilled). The counter flow chillers
work by sending the cold water and hot wort in opposite directions through tiny metal channels embedded in metal plates. This chills the wort down extremely quickly and efficiently.