Basically, there are three different types of paper sizing– strong sized or hard sized, weak sized or slack sized, and unsized or waterleaf.
Strong-sized or hard-sized papers include all the coated fine art paper used for creative purposes. Weak-sized or slack-sized paper, often referred to as soft-sized paper, includes newsprint papers along with non-waterleaf papers whose degree of absorbency is relatively high.
Finally comes unsized paper or waterleaf paper, which includes filter paper and blotting paper.