Carbohydrates are classified into three categories:
Monosaccharides (glucose, galactose, fructose) are the sugars found in milk and fruits.
Disaccharides (sucrose, lactose) might be found in table sugars and milk sugars. They are a combination of a two simple sugars.
Polysaccharides (complex carbs, starches, fibers) come from whole grains, vegetables, nuts, some fruits and legumes. These are your complex carbs.
When you link different kinds of sugars together, you will get different kinds of products. For example when you combine glucose and fructose you get sucrose (table sugar).
You'll read a lot about essential amino acids and essential fats but what you won't read about in any fitness manuals or hear from any fitness experts is the essential carbohydrate. That's because there is no such thing. There are just different carbohydrates with different properties that affect your energy levels.
Here's a fact for you: Our bodies can only absorb monosaccharides (simple sugars).
[STOP and realize that complex carbs are just a series of 3 or more simple sugars bound together. As mentioned, complex carbs go by the alias, polysaccharides.]
This means if you eat a complex carb, your body will break down that complex carb into simple sugars and ultimately into blood sugar which can be used for many different functions. Depending on the carbohydrate you just ate and other factors, these carbohydrates will have different effects on blood sugar levels. Specifically how fast they rise and fall.
This ability of a carbohydrate to to raise blood sugar fast or slow is called the glycemic index (GI). The GI was created to track various foods effects on blood sugar at different rates.