I rarely focus on single nutrients because I believe that when a person consumes a balanced diet they tend to get adequate amounts of everything their body needs.  However, many people, especially women, don't get enough of a key nutrient in their diet — iron. 

Why do we need to consume iron? — Iron plays a key role in the delivery of oxygen to cells as part of red blood cells.  It is also involved in many of the body's chemical reactions and immune function.

Signs of deficiency — Fatigue, cravings to eat ice (weird, I know), pica (craving non-food items such as dirt or chalk), getting sick frequently, decreased athletic performance, having trouble with mental tasks.

High-risk for deficiency groups — Infants, menstruating women, pregnant women, elderly, persons with certain chronic medical conditions, athletes in high-impact sports (e.g. running), vegetarians/vegans, persons with eating disorders

Food sources of iron — There are 2 types of iron in our food, heme and non-heme.  Heme iron is primarily found in animal products, while non-heme iron comes from plant sources.  In general, heme iron is much better absorbed (which is why vegetarians, especially vegans, are at higher risk for iron deficiency anemia). 

• Good sources of iron: meat, seafood, beans, egg yolks, dried fruit, cereal, enriched grains, vegetables

Iron isn't always well-absorbed from food or supplements, so it's important to consistently include it in your diet.  And just because some iron is good doesn't make more iron better.  Excessive supplementation with iron can cause toxicity, so don't overdo it.  Like I always say, food first.  As long as you are including iron-containing foods in your diet you should be fine.  If you fall into a high-risk category for deficiency you might consider asking your doctor to check you for anemia to see if you'd benefit from supplementation. 

Source: http://www.nutrition411.com/content/iron