Water heaters

Water heaters are a very common source of providing hot water. They are also very reliable, but will cause problems eventually, especially with hard water. The water heater is a tank of water that is heated up using it's own source of energy, as opposed to the indirect fired or the tankless coil, which uses an external source. Most water heaters use either gas or electric. There are oil fired water heaters as well, although much less common.
The one thing to always keep in mind with a water heater, is that it will eventually leak. It might, of course, be replaced before it does, but generally a water heater is repairable as long as it holds water. It is a good idea to plan ahead if necessary in order to prevent serious damage to your home when this leak happens, because you won't get a warning.
If your water heater is in an unfinished basement, then it shouldn't be much of a problem. The leak doesn't generally blow out and flood a basement, but it might get some storage boxes wet before it is found. The real problem is when the water heater is in a closet or on the second floor. Anywhere that a lot of damage can happen with a relatively small leak. For these cases, a drain pan is needed. The drain pan is placed under the water heater and piped to a safe location. Then, when the leak does happen, it won't damage anything around it.

Another common piece to all water heaters is the relief valve. The relief valve is water heater specific and is sensitive to pressure and temperature. The pressure is set to 150psi. This can sometimes cause problems in a public water system when the pressure gets very close to this. The relief valve also needs to be piped to within 6 inches of the ground to prevent damage or injury in the event of a blow off. Many older water heaters were not piped to the ground. New installations require this and it's a good idea to pipe the old ones correctly.

Gas water heaters can be very dangerous. Never work with a gas appliance unless you are qualified to do so. The gas burner of a water heater is very similar to a gas boiler, but if you're not experienced in working with gas, call a professional.
The electric set up in an electric water heater is mostly universal. It consists of 2 thermostats(upper and lower) and two heating elements. These elements burn out if energized without being completely submerged in water, so the water heater needs to be turned off anytime the water system will be drained down. Troubleshooting a water heater requires knowledge of how electric works and also should not be attempted by an unqualified person.