Gingival recession, also known as receding gums, is the exposure in the roots of the teeth caused by a loss of gum tissue and/or retraction of the gingival margin from the crown of the teeth. Gum recession is a common problem in adults over the age of 40, but it may also occur starting in adolescence, or around the age of 10. It may exist with or without concomitant decrease in crown-to-root ratio (recession of alveolar bone). Classification
Various classifications have been proposed to classify gingival recession, Miller’s classification system being the one that is most widely followed. Many cases which are encountered in daily clinical practice cannot be classified according to the criteria of the present classification systems. Kumar & Masamattis classification system gives a comprehensive depiction of recession defect that can be used to include cases that cannot be classified according to present classifications. A separate classification system for palatal recessions (PR) has been given. A new comprehensive classification system classifies recession on the basis of the position of interdental papilla and buccal/lingual/palatal recessions. Kumar & Masamattis classification system tries to overcome the limitations of Millers classification. Causes
There are many possible causes for gingival recession:

By far the most common cause is gum disease (periodontal disease). Overaggressive brushing also causes gum recession. One systemic review of the literature concluded that "The data to support or refute the association between tooth brushing and gingival recession are inconclusive," although aggressive or forceful brushing was not specifically addressed.
Because the changes in the condition of the gums from one day to another are minimal, patients get used to the gums appearance and tend not to notice the recession visually. Receding gums may remain unnoticed until the condition starts to cause symptoms. The following signs and symptoms may indicate gum recession:

Tooth mobility
Dentin hypersensitivity (over-sensitive teeth) - short, sharp pain is triggered by hot, cold, sweet, sour, or spicy food and drink. If the cementum covering the root is not protected anymore by the gums, it is easily abraded exposing the dentin tubules to external stimuli. Teeth may also appear longer than normal (a larger part of the crown is visible if gums are receding)
The roots of the tooth are exposed and visible
The tooth feels notched at the gum line
Change in the tooth’s color (due to the color difference between enamel and cementum)
Spaces between teeth seem to grow (the space is the same, but it seems larger because the gums do not fill it any more)
Cavities below the gum lineIf the gum recession is caused by gingivitis, the following symptoms may also be present:

Puffy, red, or swollen (inflamed) gums
Gum bleeding while brushing or flossing
Bad breath (halitosis)In some cases, it is the treatment of gingivitis that reveals a gum recession problem, that was previously masked by the gums swelling.