In short, carbohydrates are foods that provide you with energy - carbohydrates being your primary energy source. Second being fats and finally protein. Hopefully, breads, cereals, rice, pasta, fruit and vegetables being your main sources of carbohydrates throughout the day.
Broken down through the digestive process, carbohydrates are metabolised into Glucose - some going to fuel brain activity and most of the remainder stored in the liver and muscles as Glycogen.
There are three categories of carbohydrates (please click on each to find out more) - Monosaccharides (simple sugars), Disaccharides (dual combination of simple sugars), Polysaccharides (chain of simple sugars, Starches). Simple sugars and starches are commonly referred to as simple carbohydrates and complex carbohydrates respectively. Simple carbohydrates are foods that give a quick increase in blood sugars, complex carbohydrates giving a slow increase in blood sugars. Most food items are now labelled with a ranking system (Glycemic Index) to indentify this.