IC1 and IC2 are 741's which form the preamplifier stage. IC3 which is also a 741, along with Q1, Q2, Q3 and Q4 form the power stage.
IC1 is an op-amp configured as a voltage follower that isolates the CD player from the rest of the circuit. IC2 is an inverting amp which provides the voltage gain for the circuit. The potentiometer makes the voltage gain variable, thereby giving rise to volume control.
Q1, Q2, Q3 and Q4 form a complementary symmetry (push-pull) Darlington
connection output power stage.
The Darlington is just another name for connecting two BJT in series
as shown to increase the effective current gain to 
.
Q1 and Q3 are npn which are
on during positive voltage swings and off during negative
swings. Q2 and Q4 are pnp which are off during positive
swings and on during negative swings.
It is important to determine the power dissipated in the transistors
in order to decide what type of transistors to use.
D1, D2, D3 and D4 are
diodes which act to bias the push-pull output so that it is
on the edge of being on with zero input signal, thereby
reducing cross-over distortion.
R1 and R2 isolate the op-amp output from the power supply.
R3 and R4 act to inhibit thermal runaway by
reducing the value of VBE when the junction current (and
thus junction temperature) increases.
We could have connected the
push-pull circuit directly to IC2, however, to reduce distortion
an flatten frequency response, we included IC3 along with the
feedback connection. The overall effect is then to have
a voltage follower with the characteristics of an op-amp,
but with its current drive capabilities increased by the
addition of Q1 through Q4.
