Paper weights and types can be very confusing. What follows is a general guide that can help you understand the sizes and terms and to associate them with items that you are familiar with.
Click image for a larger view.
There are two basic systems in use: The US System (Traditional English System), which sizes the weight in pounds, and The Metric System, which sizes the weight in grams/meter squared (gm/m2), or commonly referred to as grams per square meter (gsm).
The U.S. system is more difficult to understand because the same number in pounds can be used for the thicker card stock or the thinner text papers. In the U.S. system, a paper’s weight is the weight, measured in pounds, of one ream. But every type of paper has its own size of ream! The metric system however is standard across all weights of papers. It measures a fixed size–one square meter
Grams per square meter (gsm) gives a much better idea as to the actual thickness of a paper. As a general rule, anything 10-35 gsm is of tissue consistency; 35-70 gsm is lighter textweight, 70-100 gsm is medium textweight, 100-120 gsm is heavy textweight/light cardstock, 120-150 gsm is regular cardstock weight, 150-200 gsm is heavy cardstock, and greater than 200 gsm is super heavy cardstock.
For an example, lets compare two papers–a 65 lb Cover (a very common cardstock) and an 80 lb Text. Using the U.S. system, you might think that the 80 lb Text is a thicker paper–after all, it weighs more, right? But a Text weight paper has a ‘feel like a letterhead or stationary paper. Cover weight (cardstock) will have a ‘feel’ like a business card or post card. And if you had a sample of each in your hand, you would discover that there is a difference in thickness–the 65 lb Cover would be thicker and stiffer than the 80 lb Text. Using the Metric system however, these two papers would actually be 175 gsm (65 lb Cover) and 120 gsm (80 lb Text).