Proteins are the building blocks for your muscles, organs, tissues, etc. Fats are necessary for long term energy storage, they carry certain forms of vitamins and nutrients (Vitamin E, for example) and your body needs fats for many of its metabolic processes.

Carbohydrates are the fuel that your body uses for energy. Your body prefers to use carbohydrates for energy, though it can also use fats and proteins, which is why "low carb" diets work. They are easier to process for energy than either the fats or the proteins and your body gets more energy "bang for the buck" with carbs.

On the molecular level carboydrates are ring molecules. A "simple" carb, like glucose, is one ring of six carbon atoms. Just like the ring on your finger. Table sugar, sucrose, is made up of two of these simple sugar rings. Starches and "complex carbohydrates" are made up of many of these rings. This is why your body takes longer to digest the complex carbs, it has to break them down into the individual rings first and then use them for energy.

This quick digestion is why your blood sugar will leap sky high when consuming a meal of simple carbs. Your body can use them almost immediately. Eat something with a lot of sugar, such as a soda, and bam! Sugar is immediately pumped into the system and your blood suger levels leap. Insulin is secreted into the blood and the insulin causes the sugar levels to drop, along with your energy.

A meal of complex carbs takes longer to digest and as a result doesn't slam sugars into your system, rather the complex carbs provide longer term energy. Fats and proteins take even longer to digest and convert into the carbs your body needs.

Extreme low-carb diets, such as they first part of the Atkins Diet, are not recommended for anything other than short term use. Even the Atkins diet brings you back into carbs after than first section, but they are the complex carbs. Simple carbs are almost completely abandoned.

As far as energy goes, though, your body only needs so much at one time and it can only store so much at one time. Someone exercising regularly or working at hard manual labor need more carbs than someone who isn't, but there are still limits to how much they can use. Those extra carbs are stored in two ways.

One way is conversion into glycogen, a type of starch, which is then stored in the muscles for near term energy reserves. The other way they are stored is fat. The body stores excess carbohydrate as fat after it has used what it needs for energy and glycogen storage. (This is a little over-simplified, but close enough.)

In addition to being converted to fat, excess simple carbs over a long period of time can lead to a variety of health issues, diabetes and heart issues being among them.

So what foods are the simple carbs? Any kind of sugar, rice, white flour, potatoes, Rice Krispies, Corn Flakes, etc. These are all "high glycemic index" (GI) foods. The GI compares foods to a glass of glucose and measures how fast those foods will spike your blood sugar.

Fruits tend to be in the medium GI range and most veggies are pretty low. Go to Google.com and look for "high glycemic foods" or "glycemic index" and you'll find charts with examples of various foods and their ratings.

Your body prefers to get its carbs slowly and steadily and works best that way. it doesn't need a ton of them at one time. You may have heard of the "eat several small meals" plans? This is one of the reasons they work. Your body has the time to properly process the carbs and so uses them, rather than storing them as fat.

Want to gain weight fast? Eat one big meal a day, preferably in the evening, and sleep afterwards. That's how the Sumo wrestlers of Japan do it.