Plaque that stays on your teeth can harden under or above your gum line into tartar (calculus).
When sugars and starches aren't cleaned off your teeth, bacteria quickly begin feeding on them and form plaque. It's due to eating a lot of sugars and starches and not cleaning your teeth well.
Dental plaque is a clear sticky film that coats your teeth. Cavities are caused by tooth decay - a process that occurs over time.