The type of paper used for difference purposes is expressed in weight. In the countries following the European system, it is grammes per square metre (gsm or g/m2 or simply g) and in the US system it is Pound (lb).
The paper used for writing/printing (letterheads and such stationery) is generally between 60 to 120 gsm (20 to 70 lb) and is called the paper or text stock. Anything heavier than this is considered the card stock, i.e. the paper used for business cards, postcards, greeting cards, etc.