Tooth decay is damage to a tooth surface(enamel) which further leads to cavity by acids released
from bacteria.

The damaged structure of tooth is removed and filled with the cements like GIC, Composite based on the amount of tooth structure damaged. 

In case of deep decay’s gic is placed as first layer to resist the pulpal irritation upon that composite is placed and if decay is very close to pulp than MTA or DYCAL is placed ad restored with temporary cement and if no pain persists than permanent restoration is done. The teeth fillings are done restore the teeth shape and function.