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.