Sodium carbonate is the inorganic compound with the formula $N
The anhydrous form is the powder that you find in chemical supply stores, swimming pool shops and hardware stores. It has no water molecules in the crystals, but it is hygroscopic and will absorb a little water from the air. If you store it in a dessicator it will remain dry, but if you keep it open for long it will absorb water and turn into a decahydrate. This is the form that is sold as washing soda. It is the most common use for sodium carbonate.
It is also used as a pH adjuster, anti-caking agent and manufacturing aid for applications like glass making where it acts as a flux to lower melting temperatures. It is also used as a food additive, water softener and in taxidermy.
Baking soda (sodium bicarbonate) is NaHCO3, and chalk (calcium carbonate; CaC