Basically,
carbohydrates are just single
sugar molecules repeated over and over again. A long chain
polymer of glucose, to be specific.
Plants store carbohydrates as
starch and possibly
cellulose (that tough, hard to break wall-like stuff in celery that gets stuck in your teeth), while
animals store carbohydrates as glycogen.
Glycogen is found mainly in muscle and occaisionally the
liver. If found in excess, the body sometimes converts carbs into
fats or oils.
When these long chains of glucose are digested, they are first broken down by the body into many maltose units. Maltose is a dissacharide molecule made up of two glucose molecules bonded together. After this, the body breaks down the maltose units into single glucose units, clearing the way for glycolysis (the first stage of cellular respiration), and eventually providing us with energy. Don't let the big words fool you, it's really simple. Just lots and lots of sugar arranged in funny ways.