Automatic circuits work on the philosophy that some external/automatic switch turns power on and off to a circuit. This can be a photocell or timeclock that controls a set of relays or contactors which allow or disallow current to flow in a circuit.
A photocell takes in light from a photovoltaic eye, and opens/closes a coil inside the photocell. It does this completely independently of human interaction. A timeclock has a constant clock that ticks and spins a dial which trips a lever to engage or disengage a circuit to allow current to flow through it.
There are some dangers associated with working on automatic circuits. Chief among them is the knowledge that at a certain time this circuit may become energized while you're working on it. If you're working on a parking lot pole light at sundown and a photocell decides to kick on because it's no longer sensing photons in it's eye, the circuit could turn on while you're hand is in the hand-hole.
Always shut power off to a circuit before you work on it, especially when working on automatic circuits. The tricky part about automatic circuits is knowing which breakers to shut off. Most automatic circuits have a photocell or some sort of timer that controls the circuit. We call this circuit the "control circuit." By shutting this breaker off, the photocell or timeclock will no longer engage, but this does not mean that the other circuit feeding the light/load is off. There are separate circuits that leave the panel, go to a contactor or relay, then go out into the field. These breakers could still be on if you have a normally closed contactor or relay feeding the load. You should always test the circuit before working on it, but taking the extra time to shut the load circuits off as well as the control circuit is a great habit to get into.
In this video I show how contactors and ice-cube relays work, and how to wire them. I also explain the difference between normally-open and normally-closed contacts.