Gum disease is caused by plaque, which is a film of bacteria that forms on your teeth. Plaque is the primary culprit for cavities, and when it's not removed, it turns into tartar. Everyone has plaque -- we can't avoid it; our mouths are constantly full of bacteria. But we need to remove that plaque daily by brushing and flossing. If we don't, plaque can irritate the gums and lead to cavities and gum disease.
Plaque builds up under the gum line and can cause infections that will make your gums feel tender and possibly bleed. This condition is called gingivitis. If gum disease becomes extremely severe, it's called periodontitis and will need to be treated by a professional. In a worst-case scenario, periodontitis could mean losing your teeth. This is why dentists are so adamant about brushing and flossing regularly (brush twice daily with fluoride toothpaste, and floss once per day).
Brushing and flossing should be done daily over the course of one's life to prevent gum disease later in life. As people age, arthritis can make it difficult to floss and brush properly, in which case a dentist may need to provide tools that will make the task easier. Also, dentures should be fitted correctly; when they don't fit correctly, plaque forms more easily and is harder to reach.