Be sure to read the "basic definitions" lesson to review atoms and molecules.
Carbohydrates are made up of sugars, starches, and fibers. Sugars are simple carbohydrates. Starches and fibers are complex carbohydrates, but starches and fibers are made up of sugar molecules, which can be confusing. The simplest, smallest molecule of these sugars is called a monosaccharide. These sugars are what as known as simple sugars, and the most common are known as glucose, fructose, and galactose.
Glucose is the key carbohydrate in the body for energy. The blood carries it, and that is why you may hear about diabetics talking about "blood sugar." They test their blood to make sure the glucose levels in their blood aren't too high. Plants produce glucose via photosynthesis. Usually, though, in our food it is found as part of a disaccharide (what you get when two sugar molecules combine, "di" means two) and starch.
Fructose is sweeter than glucose, and it is the sugar that is in fruits and honey. Galactose is actually a part of lactose, which many people refer to as "milk sugar." If you hear someone say they are "lactose intolerant," they often consume something before they eat dairy products so this sugar doesn't bother them. Remember these are still monosaccharides.
When two sugar molecules combine and become disaccharides, we get other sugars. Maltose is two glucose molecules together. It is formed when a starch is broken down. If you eat an unsalted cracker, breaking down the cracker with your teeth and mixing it with saliva, you may notice a slightly sweet taste to the mush if you keep it in your mouth before swallowing it. That sweetness is maltose. If glucose and fructose are linked, you get the white sugar we use in baking and our coffee. This sugar is called sucrose. If you link glucose to galactose, you get the lactose we referred to earlier. If you are ever very thirsty or haven't eaten in awhile and sip some milk, it will usually taste sweet. This is lactose. Incidentally, it is the only sugar in animal-based foods.
The above sugars, both the monosaccharides and disaccharides, are known as simple carbohydrates.