The anode rod in your water heater has a very important role and should be frequently checked. By being made of a metal more active than that used for the design of the tank, the anode will sacrifice itself to protect the tank.
Once the anode is completely consumed, the tank no longer has any protection and will begin to rust.
There are two types of anodes:
The best known are the sacrificial anodes. In most cases, the anode is composed of magnesium, aluminum and more rarely zinc. The size and length of the anode are very important because they will affect is lifetime.
Demand current anodes use a totally different technology to protect the tanks. The forced current anode uses an external power source (current rectifier) to passivate the tank, which protects it against corrosion. Finally, these anodes are well known for their ability to solve sulfur smell.