Carbohydrates are chemicals composed of one or more simple sugars. Includes sugars, starches, and cellulose, the main form of dietary fiber.
Carbohydrates are your body's primary source for energy. Carbohydrates include sugars, starches, and cellulose. Your body must break down carbs in order to turn them into glucose.
The reason too many carbohydrates (particularly the wrong kind of carbohydrates, such as refined/processed/concentrated carbohydrates) are not good for you is that your body can convert them into body fat to use (or not use) later. This applies to all carbohydrates.
Too many carbohydrates, in particular refinedprocessedcarbohydrates, are a major cause of weight gain, obesity, heart disease, insulin resistance, type 2 diabetes, and other diet related diseases.
For a listof refined processed carbohydrates see the page link, further down this page, listed under Related Questions.