A Darlington transistor, sometimes called "Darlington pair" is a transistor circuit that is made from two transistors. Sidney Darlington invented it. It is like a transistor but it has much higher ability to gain current. The circuit can be made from two discreet transistors or it can be inside an integrated circuit. The hfe parameter with a Darlington transistor is each transistors hfe multiplied together.
The circuit is useful in audio amplifiers or in a probe that measures very small current that goes through water. It is so sensitive that it can pick up the current in skin. If you connect it to a piece of metal, you can make a touch-sensitive button.