The TIP series of transistors are NPN Darlington transistors (2 transistors stacked inside the package) originally designed for audio amplifiers.
They are also excellent motor control transistors! The have a low trigger voltage, so you can run them from an Arduino or other microcontroller, and pass lots of volts and amps through them.
TIP 120 is the first in the series, but all behave in the same way, with different specs for amperage.
