Water heaters all have a sacrificial anode in them. The anode protects the steel used in manufacturing of the tank from corrosion caused by electrolysis (different metals in water). The anode gets used up so the steel tank does not start to corrode or rust through and start to leak! Lets say you purchased a water heater with a 6 year warranty. The manufacturer figures the anode will last for about 5 years average use, and the tank will rust through about a year after that. Hence a 6 year warranty. A 12 year guaranteed water heater usually has 2 anodes in it, so a longer warranty! I recommend you check the anode(s) in your tank every 2-3 years with normal family use. If you have a water softener the anode will be used up much faster because of the salt used in the system. If this is your case check it every 1-2 years.

I know this is an almost unheard of thing but feel free to ask me about it and I will help. Pic shows a new anode on top and used up one on bottom. Usually made of magnesium or aluminum which are softer than steel and get corroded first rather than the steel tank.