This 12 Volts transformerless power supply take advantage of the fact that a Zener diode is also a normal diode that conducts current in the forward direction. During one half wave, the current ﬂows via D1 through the load and back via D4, while during the other half wave it ﬂows via D3 and D2. Bear in mind that with this circuit (and with the bridge rectifier version), the zero voltage reference of the DC voltage is not directly connected to the neutral line of the 230-V circuit.
This means that it is usually not possible to use this sort of supply to drive a triac, which normally needs such a connection. However, circuits that employ relays can benefit from full-wave rectification. The value of the supply voltage depends on the specifications of the Zener diodes that are used, which can be freely chosen. C2 must be able to handle at least this voltage.
The amount of current that can be delivered depends on the capacitance of C1. With the given value of 220nF, the current is approximately 15mA. A final warning: this sort of circuit is directly connected to mains voltage, which can be lethal. You must never come in contact with this circuit! It is essential to house this circuit safely in a suitable enclosure.