Our mouths are full of different bacteria (germs). Most bacteria are helpful, but some of the bacteria that live in plaque can cause tooth decay and gum disease.
Tooth brushing sweeps away plaque and the bacteria living within it. If not regularly brushed away, over time plaque can harden into a yellow/brown tartar (also known as calculus). It is usually seen just around the gum line.
While plaque can be easily brushed off with a toothbrush, tartar can only be removed by a dental professional.