Dishwashers have come a long way in the past 40 years. In the early days dishwashers were solidly built, but used tons of hot water and were very noisy. In recent years, the most interesting advances have come from the use of more sophisticated electronics.
Some of these dishwasher parts are capable of "seeing" the amount of dirt in the water, and can therefore change the amount of wash time needed to clean the dishes. In addition, many dishwashers now have a stainless steel interior which can last for 20 or more years.
Some people ask what the most common dishwasher problems are. The most common problem by far is that the dishes aren't getting clean. There are several dishwasher parts that can contribute to this problem. The main part is the water inlet valve. Most dishwashers have a timed-fill cycle. That means they allow water to enter the machine for a certain length of time. However, if there is a clog in the water line, or if the water inlet valve diaphragm has deteriorated, the water volume isn't enough to fill the dishwasher adequately. This can lead to "pump starvation" which is just a fancy way of saying there isn't enough water in the unit for the pump to spray on the dishes. This dishwasher part can't be fixed, it must be replaced.
Another common dishwasher part that fails is the pump impeller assembly. Most dishwashers have a pump motor that is mounted vertically, just beneath the center of the lower spray arm. Attached to the top of the motor are usually two impellers; one to pump the water up at the dishes, the other to pump the water out. The motor reverses direction to accomplish those two tasks.
Finally, it may be that all of the dishwasher parts are fine. It might be that you need to check the temperature of the water reaching the appliance. If the water temperature is less than 110 degrees you will often have washability problems. And, use a good powder detergent. Historically, the liquids and gels have not performed as well as the powders and tabs.
Dishwasher parts can be expensive depending on the brand. Be sure to always replace the ones in your appliance with genuine parts. Click the link on this page to find the Internet's largest supply of dishwasher parts.