Biasing the push-pull follower to eliminate distortion
A BJT does not begin to conduct until the base-emitter voltage reaches somewhere around 0.6V.  Therefore if the two biasing "diodes" weren't there there would be a range of input voltages from about -0.6V to +0.6V where both transistors are off.

In order to improve this situation each transistor is biased by a diode-connected transistor (as shown) and preferably mounted on the same heatsink as the output device so the Vbe drop tracks with temperature.

This means that the dead zone is much smaller.  In the zone right around zero, the amplifier doesn't have to supply much power, but the transistors still might be close to cutoff.  For that reason the 100 ohm resistor couples the input signal right to the output to further improve the zero crossing performance.

Both devices are emitter followers, and the top NPN device conducts for positive signals while the bottom PNP device conducts for negative signals.