Calories are found in carbohydrates, proteins and fats.

Carbohydrates are an interesting bunch concerning human metabolism. (See http://www.faqs.org/nutrition/Ca-De/Carbohydrates.html for a nice chart on comparing carbohydrates) 

Simplistically, there are three basic building block of Carbohydrates called Monosaccharides.  These simple sugars are:

Glucose = most common form of carbohydrate, used as an energy source in the body

Fructose = primary sugar found in fruits, also is found in honey and high-fructose corn syrup (in soft drinks)

Galactose = found in milk and other dairy products

In the body, fructose and galactose are broken down into glucose and used as fuel.

The building block monosaccharides can be combined to create novel Carbohydrate units called disaccharides and polysaccharides.  For instance, the simple carbohydrates disaccharides are:

sucrose — common table sugar = glucose + fructose

lactose — major sugar in milk = glucose + galactose

maltose — product of starch digestion = glucose + glucose

Polysaccharides are complex carbohydrates consisting of repeating linked (polymers) mono- or di-saccharides simple sugars:

Starches = amylose and amylopectin - found in plants like potatoes and rice

Glycogen = polymerized glucose - used as a stored form of glucose in the body

Cellulose = (differently) polymerized glucose - plant "fiber", not digested by humans

So, for the most part in my simplistic biochemistry lesson, nearly all carbohydrates are ultimately used by the body as glucose.  A carbohydrate is a carbohydrate.  A sugar is a sugar.  Whether your sugar is from refined white sugar, less refined brown sugar, honey or fructose containing colas, the human body treats them all identically.

The problem of obesity is multifaceted.  Concerning high fructose colas, there is a lot of carbohydrate in a 12 fluid ounce Coca-Cola:  39.9 grams of carbohydrate (13% of recommended Daily Value based on a 2000 Calorie Diet), equaling 143 calories in a can of coke, which is equivalent to 9.5 teaspoons of table sugar per 12 fluid ounces.(http://www.calorieking.com/foods/food/carbs-calories-in-coca-cola-sodas-soft-drinks-coca-cola-coke-classic_Y2lkPTE1NiZiaWQ9MjI0JmZpZD0xMDc4NTEmcGFyPQ.html)  Packing that many calories into a small volume which leaves room for even more consumption of calories is one factor in obesity.

Then there is the so-called Glycemic Index food effects.  In this scenario, foods are compared to a serving of white bread for their effect on the pancreas to release insulin.  The higher the Glycemic Index the food is, the more insulin the pancreas secretes.  The higher the jolt of insulin the more likely a compensatory hunger response may occur.  Think of the steroetypical Chinese food joke about eating a meal and then being hungry again an hour later.  This is one of the effects, theoretically, from high Glycemic Index foods.  The other theorized effect of long term ingestion of high Glycemic Index foods is increased stress on the pancreas from continuously kicking into high gear abruptly contributing to development of diabetes. 

And we haven't talked yet about people who turn to food either as a coping mechanism of from genetic disorders or physical illnesses in considering the causes of obesity. 

The bottom line concerning obesity is the number of calories from all food ingested in 24 hours minus the number of calories used/burned off.  If you ingest more calories than you use as fuel per unit time then the remaining calories will be converted to glycogen (storage form of glucose) and fat.

Here's another tip for anyone thinking of starvation diets to lose weight.  The body will first use up glucose, then glycogen stores, then slow down metabolism (burn less calories) to conserve function, then preferentially metabolize (break down) muscle before burning fat as well. Not only do you burn calories at a slower rate, ie lose weight slower, but you also lose muscle mass.  That's bad in anyone's book.