Carbohydrates, sometimes simply called carbs, are the most abundant biological molecules, and fill numerous roles in living things, including the storage and transport of energy (as glycogen and starch).
The basic carbohydrate units are called monosaccharides, such as glucose, galactose, and fructose.
There are two types of carbohydrates, simple carbohydrates and complex carbohydrates. Simple carbohydrates are also called simple sugars like the table sugar which is almost 100% simple carbohydrate. In some foods like fruits and milk, simple carbohydrates are present. Simple carbohydrates present in fruits and vegetables are better than sugars as the former also contain vitamins, fiber, other nutrients (like calcium in milk). Complex carbohydrates are also called starches, and their main sources include grain products like breads crackers, pasta, and rice. Processing of complex carbohydrates reduces their nutritional value. Generally, complex carbohydrates are preferable to simple carbohydrates. However, a combination of select foods from both groups are preferable.
Foods containing carbohydrate have remained significant since time immemorial. Many foods are good source of carbohydrates. According to Harvard School of Public Health, "Carbohydrates come from a wide array of foods - bread, beans, milk, popcorn, potatoes, cookies, spaghetti, corn, and cherry pie. They also come in a variety of forms. The most common and abundant are sugars, fibers, and starches. The basic building block of a carbohydrate is a sugar molecule, a simple union of carbon, hydrogen, and oxygen. Starches and fibers are essentially chains of sugar molecules. Some contains hundreds of sugars. Some chains are straight, others branch wildly."
- Carbohydrates - a page from the website of Harvard School of Public Health
- Types of Carbohydrate - a page from the American Diabetes Association