Pica is an appetite for non-nutritive substances (e.g., coal, soil, chalk, paper etc.) or an abnormal appetite for some things that may be considered foods, such as food ingredients (e.g., flour, raw potato, starch). In order for these actions to be considered pica, they must persist for more than one month, at an age where eating such objects is considered developmentally inappropriate. The condition's name comes from the Latin word for the magpie, a bird which is reputed to eat almost anything. Pica is seen in all ages, particularly in pregnant women and small children, especially among children who are developmentally disabled, where it is the most common eating disorder.
It is often cigarette butts that are consumed, presumably for the nicotine content. Most developmental centers have a no-smoking policy[citation needed] due to this fact, and developmentally disabled persons out in the community are often at risk of sickness and/or choking on the litter they find.
It is much more common in developing countries and rural areas than elsewhere. In extreme forms, pica is regarded as a medical disorder.
Pica in children, while common, can be dangerous. Children eating painted plaster containing lead may suffer brain damage from lead poisoning. There is a similar risk from eating dirt near roads that existed prior to the phaseout of tetra-ethyl lead in gasoline or prior to the cessation of the use of contaminated oil (either used, or containing toxic PCBs) to settle dust. In addition to poisoning, there is also a much greater risk of gastro-intestinal obstruction or tearing in the stomach. This is also true in animals. Another risk of dirt eating is the possible ingestion of animal feces and the accompanying parasites.

More Info, Source and Links:
http://en.wikipedia.org/wiki/Pica_%28disorder%29