First, let's consider the water in your pond in the same relation as the blood in your body. The blood carries oxygen to the vital organs and carries away unwanted carbon dioxide and other waste. Based on this simple concept, we can make the following annologies:
The Heart: The heart of your pond is the pump. The pump pushes the blood through the system, oxygenating the water and transporting waste to a waste processing facility.
The Lungs: Consider anything that oxygenates the water and breaks up potenitally toxic gases in your pond the lungs. The best lungs are your waterfall or stream, where the blood is exposed to the air and is mixed up in a turbulant manner. The more turbulant the blood flow, the more oxygen the blood will absorb, and the more potentially toxic gases it will expell. An aerator, or bubbler, will also oxygenate the blood, but not nearly as effectively as water falling through the air and breaking over rocks.
The Gastro-Intestinal System: Your filter system is your pond's intestines and related organs. The skimmer box, where the pump is housed, does the initial work of catching waste and processing it, not unlike your stomach. If you have a bottom drain (and you should), more waste can be removed from the system, thus making the entire system healthier. Inadequate filtration can lead to intensive and constant maintenance problems, much like not having fully functioning kidneys necessitates dialysis. Your own liver produces vital enzymes to process what you eat and convert it into either energy or waste. The filter media in your pond filter does much the same, growing beneficial bacteria to convert Ammonia to Nitrites, and Nitrites to Nitrates (which are beneficial to plant growth), as well as collecting solid waste for future disposal.
Fat and Calories: Fortunately, fat and calories in a pond don't build up like they do on my hind end. Instead, the energy or calories, are consumed by the plants. If you don't have enough plant coverage in your pond, you are likely to experience green water, or pea-soup algae. (See "HELP! My Water's Green!") Therefore, we can say that a healthy pond requires diet (moderate intake of solids) and exercise (plant coverage to consume excess calories.)
Nervous System: Electricity is the conduit for everything that runs in your pond, and the GFI outlet is the brain. Your pump, lights, aerator, spitters are all run by electricity. If something goes amiss, the GFI will shut the power off to whatever circuit it detects a fluctuation in. I cannot stress enough now important it is to install GFI receptacles for running anything and everything in and around your home. Most ponds can be run entirely on one 20 amp circuit, as most everything you install will be extreamly low voltage, but if you are nuts about lighting up your whole back yard, you will want to have a qualified electrician dedicate more than one circuit to the system. Additionally, there are times when our brain tells us what to do, like breathing for example. However, there are also times when (in most cases) we can tell our brains what to do, like holding our breath. In a pond, the most common reason for you to want to shut something down is related to the pump. Maybe you just noticed a leak in your waterfall, or one of your fish got into the skimmer box by mistake. Instead of frantically pulling one cord after another until you get the right one, a better option is to have a kill switch installed at the receptacle when you have the power run. I always recommend having a minimum of one 4-gang box installed along with a kill switch. Four receptacles should meet all of your initial power needs.
Chocolate and Ice Cream: You don't really need either of these in your diet, but what fun would life be without them? The same is true for having fish in your pond. You don't need them to have a healthy, well balanced system, but what's the fun in that? Certainly it can be said that if you don't have fish, your filtration needs are substantially reduced, but if you do plan on having fish (and I'm assuming here that you do), you will need to take that into account when planning your pond size and filtration requirements.