Short answer – paper or cardboard. And in those two categories, there are a range of different thicknesses (called weights).
Long answer – the type of paper used for different purposes is expressed in weight. In countries following the European system, it’s measured in grams per square metre (gsm, g/m2 or simply g), while in the US system, it’s in pound (lb).
The paper used for writing/printing (letterheads and similar stationery) is generally between 60 to 200 gsm (20 to 70 lb) and is called paper or text stock. Anything heavier than this is considered card stock, ie. the paper used for business cards, postcards, greeting cards, etc.