What is dental decay?
Dental caries is the scientific term for tooth decay or cavities. The combination of dental plaque, bacteria and sugar produces acid that destroys the tooth's enamel and the layer under it, known as the dentine.​
Food particles stick to bacteria in the mouth to form a sticky film called plaque which adheres on the surfaces of teeth. Plaque forms especially easily in cracks, pits or fissures in the back teeth, between teeth, around dental fillings or notably difficult to clean (e.g. braces and crowns). Removing plaque effective and frequency reduces risk of decay.

Fillings

If a patient has decay in their tooth, the tooth will generally have symptoms of pain and sensitivity to hot, cold and sweets.
The standard treatment for decay is to fill the tooth. Fillings usually are made of a dental amalgam, which is a silver material, or of a composite resin, which is tooth-coloured for a better appearance.
If a cavity is large, the remaining tooth may not be able to support the amount of filling material that would be needed to repair it. In this case, the dentist will remove the decay, fill the cavity, and cover the tooth with an artificial crown.

Root Canal Therapy

Root canal therapy is used to treat a tooth that has an infection, most likely caused by decay or trauma. A tooth that is infected can involves symptoms of sharp or throbbing toothache to high temperatures, resulting in a dental abscess. Treatment involves the removal of infected nerve or pulp tissue which is then cleaned & replaced it will filling material. 
It is highly recommended to have a crown placed after treated is completed  as these teeth weaken over time.

Dentures

If you have lost your natural teeth due to damage, decay or injury, dentures can replace your missing teeth and restore your appearance and function

Dentures are removable false teeth that are worn to replace missing teeth. These artificial teeth are generally made of acrylic and designed to best match your natural teeth.