glycogen is stored sugar (i.e. carbs) in the body. it's used as fuel for the body during exercise. Carbs should actually make up about 60-70% of your daily food intake. Carbs and glycogen is the primary source of fuel for exercise. Fat is used too, but physiologically (is that a word?) it needs a lot of oxygen to get used. Long distance athletes who control their pace well, do you use a fair bit of fat as well as carbs. If there isn't enough oxygen, there will be anaerobic (ie lack of oxygen) breakdown of the carbs and glycogen which results in lactic acid build up (ouch!)

hope this helps a little