a contactor i guess is kind of a electro mechanical device, as it has larger moving parts, contactors can deal with high currents without so much fuss,
relays do not have the same moving parts, the high power relays needed to switch off many lights are more costly than a contactor,
using contactors to switch lights is pretty standard, relays can be used, but i think a contactor is better suited to handle the large currents and inductive loads you get from these lights
relays can suffer from the back EMF that is produced when the lights turn off
that causes the contacts of the relay prongs to melt shut,
this can happen to regular house hold timer switches when they are used to switch lights, as they do not have the relays needed to handle high inductive loads inside them .. they are designed for resitive loads
with a contactor when the time switch is set to on, a small current will pass to a coil that is located around the contactor rod this causes physical movement in the contactor rod due to induction
you can hear this movement easy.. its a latching sound the contactor rod inside the device moves foward into the correct position to "fill in the gap" of one of the nodes .. that breaks the circuit inside the device ..
contactors remind me of those buzzer locks some places have to let you in,
you ring the bell and they buzz the door open these electronic latching locks are very similar to the principle of a contactor
a relay is more simple it is just 2 conductor prongs in a glass envelope when a magnetic field is brought close enough, or is strong enough, it will cause the prongs to slam shut this creates a path for the current to flow , take the magnetic field away the prongs open .. rap a coil around the glass relay case, you have made a reed relay switch
sorry for this long winded answer folks but i knew something about electronics once, but its ok i think i forgot most of it now .. long live the weed