This will be a 2-hours introductory talk. After reviewing some basic concept of 3-manifold theory (such as the irreducibility of a 3-manifold, the incompressibility of an embedded surface, etc), a norm is defined on the homology of 3-manifolds (the Thurston's norm). D.Gabai combined this discovery with Waldhausen's notion of a hierarchy of a 3-mainfold, producing a theory of sutured manifold. The main ideas and some remarkable applications of this theory will be mentioned in the talk.