What does GSM mean?

GSM is an acronym standing for ‘grams per square metre’. The higher the gsm number, the heavier the paper.  

If you were to take several sheets of different types of paper (e.g. tissue paper, newsprint, printer paper and some heavy cardstock), and then cut them all into equal pieces measuring one square metre each, and then weigh each of those sheets… the weight of each sheet (in grams) would give you the grams per square metre. Obviously the thinner paper would weigh less than the heavier cardstock.

If you think of normal printer paper being about 80gsm, and a heavier cardstock (for cardmaking) being about 250 - 300gsm, then you start to get an idea of how to gauge if the paper you want to use is suitable for its intended purpose.