Water pressure between 40 and 80 psi (pounds per square inch) is considered normal in a residential setting. A home inspector will typically measure water pressure by screwing a pressure gauge to an outdoor hose bib (faucet) and opening the valve. The pressure reading in the picture below comes in at about 62 psi, right in the middle of the desired range. 

This is a good reading to see - not too high and not too low. Goldilocks would like this house.

Water pressure above 80 psi can lead to problems. Some washing machine and water heater warranties may be voided if hooked up to a house with high pressure. Flexible supply lines for washers can actually burst with high enough pressure. 

Water pressure below 40 psi can be a problem if multiple plumbing fixtures are operating at the same time. Lower pressure means lower water flow and potential inconvenience to someone taking a shower while someone else flushes a toilet. 

Water heaters are one of the most important things for daily comfort in our homes. Who doesn't look forward to that hot shower in the morning to get you going for the day? We rely on our water heaters to work day in and day out throughout years of service and rarely think about them. They are usually hidden away in the garage working hard to make hot water for daily use and slowly corroding away.

Since they are so back of mind, it's a good idea to check on them every once in a while. Catching problems early saves you in the long run, emergency service calls by a plumber tend to be a bit spendy.

Something that is easy to catch if you look for it is early signs of galvanic corrosion at the supply lines. Galvanic corrosion happens when two dissimilar metals make contact in a wet environment. The supply pipes to a water heater are a prime location for this to occur.

What causes it? Like the root of a lot of problems in a house - water. Obviously water flows through the pipes and it can act as a bridge for an electrical charge. The pipe connections have to be shielded from water. This is usually done with a dielectric union and teflon tape. In the picture above likely both of these safeguards have failed. 

This can be an easy fix if caught early. The dielectric union needs to be replaced and the connections need to be well protected with teflon tape. 
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I recommend watching this video from This Old House to learn more about this issue.