The wort chiller comprises a stack of plates with narrow channels between them. The hot wort from the kettle flows in one direction (say, top to bottom) in every other channel, and coolant flows in the opposite direction (bottom to top) in the remaining channels. Plate chillers can provide very efficient heat transfer, with the wort exiting at just a few degrees warmer than the entering coolant, and the coolant exiting at just a few degrees cooler that the entering wort.

This topic is covered in The Chemistry of Beer: The Science in the Suds, Chapter 8.