Heaters are used to keep temperatures stable above ambient temperature. Heaters are considered a cheap addition in general. Most used are glass immersion heaters and they have a temperature range of about 20-30 degrees celcius.
A heaters strength is determined by its wattage, the larger the tank, and the colder ambient temperature the higher the wattage needed to heat the aquarium. Heaters generally come in the following wattages - 50w, 75w, 100w, 150w, 200w, 250w, 300w.
New Heaters have a control to set the required temperature at, an internal thermostat keeps the water at a constant level. Heaters can also be connected to some chillers. This offers excellent temperature control.
hysteresis : its basically the difference in the ON and OFF temp of the thermostat. Eg: Heater may be set to 26C, but will only turn ON at say 25.5C and turn OFF at 26.5C. therefore the hyteresis is 1C. As far as I know most cheep heaters can have a hysteresis of up to 2-3C, so don't always but the cheapest, think that the Jager claim a 0.5C which is pretty good.
Some more expensive heaters, such as Aqua Medic, have a titanium sleeve as opposed to glass which is resistant to corrosion and and breakage.
An inexpensive heater guard made of plastic can be used to protect the heater from possible breakage.
Generally the heater is placed in the sump to avoid being seen in the display tank. Care must be taken not to place the unit in an area with too low a flow or an area that is affected by evaporation. i.e. an area in which the level drops when water evaporates such as the return chamber.
Generally tropical marine tank temperature is considered to be ideal between 25 and 27 degrees celcius.