The BZX79 series is a family of low voltage regulator diode. The diode is made in axial-lead DO-35 glass package. Regulating voltages varies from 2.4V to 75V with a total power dissipation maximum of 500 mW.
In some datasheet, the serie is labelled as zener diode, but this is not always the case as the serie relies on avalanche breakdown for higher voltage.
The BZT52 series is a good surface-mount analogy for the BZT79 series.
Applications
Diodes in the BZX79 serie are used for voltage references or low voltage stabilizers.
References
BZX79 voltage regulator diodes Wikipedia(Text) CC BY-SA