The switching action of transistors is improved by combining them in a Darlington pair configuration.
The "first" transistor acts as a signal amplifier which triggers the "second" transistor with a larger signal than the original input and thus improves its switching action. A base voltage of 1.2V is required to trigger the "pair". Darlington pairs are most commonly used in the form of Darlington transistors (eg TIP 120) where the combination is produced as a single package. These transistors are capable of handling high currents (e.g 5A) and are useful for driving motors etc.
Multiples of Darlington pairs, are available as I.C. packages e.g. ULN 2803 contains 8 Darlington pairs. N.B. I.C. Darlington pairs can usually handle only small currents typically 550 mA.