There are many different types of carbohydrates, some of which are broken down and absorbed very quickly by the body, such as simple sugars, which is not good, and some that take longer to break down, also known as complex carbohydrates. Consuming complex carbohydrates is much healthier, as the energy is made available gradually. This means the body doesn't need to store it. However, over consumption will result in excess carbohydrates being converted into fat and may lead to obesity.
Table sugar is a simple sugar and is available for energy almost immediately. Some examples of complex carbohydrates include brown rice, potatoes (not french fries!), rye bread, whole wheat bread, and some pastas. If you want to learn more about this I suggest you read up on the glycemic index (see below). Becoming familiar with diabetic is also a good way to learn about carbohydrates because anyone with diabetes really needs to eat healthy and avoid almost all simple carbohydrates.
Any food that has anything in it that ends in '-ose' means it has sugar. For example, sucrose (table sugar), fructose (found in fruit and honey), lactose (found in milk), and galactose.
Starch is a term sometimes used to refer to complex carbohydrates.
One gram of sugar/carbohydrates has about 4 calories. It's not as high as fats, but it can easily be converted into fat if too much is consumed.
About 55% of your calories should come from carbohydrates, most of those from complex carbohydrates. Sometimes carbohydrates get a bad rap, but they are vital for health and for energy, especially for energy needed by the brain.
The types of carbohydrates you should avoid are things like white bread, white rice, cookies, too much pasta, too many potatoes, and pastries. The more processed a food is the more likely it will have bad carbohydrates. The kind of carbohydrates you should consume include foods like fruits, vegetables, and whole grains.
The Glycemic Index (GI) is a method of classifying carbohydrates depending on the effect they have once ingested. If a food is converted quickly into glucose (the most simple sugar which the body easily stores or uses) then it is said that the GI of that food is high. If the food takes a long time to be broken down into glucose then the GI is low. Foods with a high GI will cause a sharp spike in insulin release which in turn makes the body convert and store the glucose as fat. This has also been shown to increase the risk of type 1 and 2 diabetes. Because the GI does not indicate how any carbohydrates are in a food we need to refer to what's called the Glycemic Load. The GL is determined by multiplying GI by the amount of carbohydrates a food contains.
Generally speaking, the more processed a food is the higher the GI and GL. The more natural a food is the lower the GL.