Heaters do exactly as their name states, heats your tank when it falls below a certain temperature.
Ideally you will purchase a saltwater aquarium heater that has the capability of maintaining a certain temperature, and will click off when your aquarium reaches the ideal temperature that you set it at.
Heaters are not all the same though, you have the choice between shatterproof heaters, and exactly how many watts you need in order to keep that tank at temperature that you desire.
Shatterproof heaters may be a bit more expensive than the others, but in the long run you will be glad that you spent a little bit more money.
Heaters in saltwater tanks are susceptible to something called salt creep, which is basically salt that has crept into crevices of anything that goes into your tank.
When your heater gets caked with salt creep, it might cause the heater to remain on and over heat.
When you remove the heater from the tank, it can and more than likely shatter.
Shatterproof heaters will combat this problem.
Generally you need at least 2-3 watts per gallon of water, luckily most heaters show the size tank that they are capable of maintaining.
One other important factor in choosing a heater for your tank is that if you separate your large heater into 2 smaller heaters, this will ensure that If one malfunctions you always have a back up and that one will not have the capacity to overheat your tank and essentially cook your fish.