Sobres (Papelería empresarial)

"pt" stands for "point"
A unit of measurement used to indicate the thickness of paper or cardstock.

• One point (1 pt) equals 1/1000th of an inch.

• For example, 14 pt paper is 0.014 inches thick, while 16 pt paper is 0.016 inches thick.
  

"lb" stands for "pound"
The number is the weight of 500 sheets (a ream) of standard-size paper

• Text weight is lighter
• Cover weight is thicker and sturdier, like cardstock

The higher the lb number within the same category, the thicker and heavier the paper feels.
Note: that 70 lb text is much thinner than 70 lb cover due to differences in base sheet size used for weighing.