TIP120 is an NPN Darlington transistor which is mainly used for general purpose amplification and medium power switching applications.
It mainly consists of an emitter, base, and collector which are the basic parts of any transistor. This is a bipolar power transistor where conduction is carried out by both charge carriers i.e. electrons and holes but main charge carriers are electrons as this an NPN transistor.
TIP120 comes with a Darlington pair in which two transistors are connected in such a way, the current amplified by the one transistor is being amplified further by the other transistor. This configuration features much higher current gain as compared to if each transistor is taken separately.
In this NPN Darlington configuration, the emitter of the first transistor is connected to the base of the second transistor which allows the current amplify.