Pica is a condition involving persistent eating of 'nonfood' substances (aside from reasons of cultural or social practice) for at least 1 month.

Some variations of pica, such as the eating of rocks or earth etc., can be linked to deficiencies of minerals etc. - which somehow leads to a strong appetite for ('inedible') substances which contain those minerals.

Many other types, such as paper eating, ice eating, or the intentional ingestion of sharp objects can't be explained by any nutritional need.

The disorder is fairly widespread, and is frequently seen, for unknown reasons, in pregnant and postpartum women - some sources say the figure could be as high as 30% (Source : University of Illinois, Chicago)

The reasons for the development of the disorder (in cases where there's no apparent nutritional deficiency) are unknown.

More info. at Wikipedia