The emitter current of Tr1 is the base current of Tr2. A change in base current of Tr1 can give a change 100 times larger in its emitter current. A change in the base current of Tr2 has a similar effect on its emitter current.
Therefore there is an overall amplification of 100 x 100 = 10000 times. This circuit is sometimes called the Super Alpha Pair.
It is often used as a power output stage.
The two transistors can come in the same package.