Paving stones can be sealed or left natural. Sealed stones will have a rich, wet look, while stones left unsealed will have a softer, more subtle effect. Depending on the product used, sealer also protects against staining, helps prevent weed and moss growth, and can lock in the joint sand.
Care should be taken when applying sealer to avoid ruining your pavers. If you follow the basic steps outlined here, you will be just fine.