In depth
GSM measures mass per unit area. A sheet of 130gsm A4 paper weighs about 8.1 grams (130 g/m² × 0.0625 m²). The number tells you nothing about thickness directly — coated and uncoated papers at the same GSM feel different because the coating compresses the fibres — but it is the standard interchange unit between paper merchants, print shops, and customers across Europe and most of the world.
In the US, paper weight is often expressed in basis weight (lb) tied to a specific parent sheet size. The same paper that a UK shop calls "100gsm offset" might be sold in the US as "67 lb text". Conversion charts exist but are inexact because basis weight depends on the parent sheet size convention.
Common GSM points to remember: 70–80gsm = office paper; 90–115gsm = leaflet/flyer inner pages; 130–170gsm = quality flyer or brochure cover; 200–250gsm = postcard or thin card; 280–350gsm = business-card stock; 400gsm+ = heavy card and packaging.