When our gums are healthy they fit nicely and tightly around the teeth and don't bleed when brushed. They should be a nice pink colour and have a stippled pattern. With poor oral hygiene the gum margin becomes red and bleeds when brushed. At this point a vicous circle may set in as the patient stops brushing because of the bleeding.
With Successful Treatment

With healing the gum shrinks and is tight around the tooth. The bone cannot regenerate and when lost is not replaceable. A problem can arise when the gums receed as the bottom of crowns may be exposed together with some root surface. Exposed roots may become sensitive due to the open structure of dentin.
As the bacteria spreads down the inside of the gum they disturb the bone and eventually start to cause the destruction of the bone. As the bone is destroyed, the tooth loses attachment and may become to move. As more bone is lost the tooth may move upto 2mm in a front to back direction. many teeth in this situation are unsaveable and must be extracted.