Gingivitis is the inflammation of the gums around the teeth. Gingivitis may be caused by a buildup of plaque and tartar because the teeth have not been cleaned properly. It can also be a manifestation of a systemic disease, as in diabetes or AIDS.
Sometimes excessive brushing or flossing can wear out the gums causing gum recession.
If gingivitis is not treated by proper oral hygiene, it can lead to periodontitis, which is a more advanced state of gum disease, that affects the bone and is harder to treat.