What causes tooth decay or dental caries?

Improper oral hygiene habits responsible for deposition of food around and in between tooth/teeth leads to plaque and tartar deposition around teeth.  This plaque contains bacteria which produced acid and this causes damage to tooth which results in black discoloration in and around teeth i.e. dental caries/tooth decay

Why does a decayed tooth need treatment?

A small decay if left untreated grows gradually with further food accumulation in and around the tooth.  Superficial decay can be restored with simple fillings where as deep decay causing pain will need root canal treatment and its protection in form of caps

How would I know that I have a cavity?

The very first symptoms is food accumulation in one location, followed by sensitivity to hot or cold and sweet food items.  Deeper cavity usually causes pain which is usually aggravated with  cold or hot stimulation. 

How to prevent dental decay?

Good oral hygiene consisting of personal care (proper brushing at least twice a day and flossing at least daily) and professional care (regular dental examination and cleaning, at least once a year) is the primary prevention against dental caries.

Whats role of diet in dental decay ?

Sticky sweet food items if one failed to remove with brushing is the main cause of dental decay.  One should avoid milk before going to bed as milk deposited around teeth will lead to dental decay behind front teeth usually in small kids