Iron-deficiency anemia is when there's not enough iron in your blood to allow your blood to do its work. Iron is what helps blood carry oxygen through the body, and without enough iron, your organs and tissues don't have enough oxygen to function at peak performance. That's why we get so tired when we're anemic -- just not enough oomph to get the job done. Iron deficiency can be caused by many things, including menstruation, not eating enough foods that are rich in iron, or gastrointestinal bleeding.