Tooth decay occurs when bacteria inside the plaque produces acid from carbohydrates (i.e. glucose, sucrose) that slowly dissolves the tooth. This leaves behind a cavity, which needs to be cleaned and replaced with a filling as soon as possible, otherwise the bacteria will reach the nerve of the tooth and cause irreversible damage.
If tooth decay is recent, it is at a stage which can be reversed through good oral hygiene and products that contain fluoride or CPP-ACP. The top of the tooth is one of the most common areas for tooth decay, as plaque often becomes trapped in the grooves and fissure decay is formed (very common in children).
Another common area where tooth decay occurs is between the teeth. It is especially important to floss between your teeth to prevent tooth decay, as it may be a bit tricky for brushes to access these areas.
Additionally, tooth decay frequently occurs along the roots of teeth, as the roots are not protected by the hard enamel that the rest of the tooth possesses. It is important that you do not brush too hard along the gum, as this will make the tooth more prone to decay since the roots of the tooth are exposed.