"Irreducible orientable compact 3-manifolds have a canonical (up to isotopy) minimal collection of disjointly embedded incompressible tori[?] such that each component of the 3-manifold removed by the tori is either atoroidal[?] or Seifert-fibered[?]"
See Thurston's conjecture for relevance.
wikipedia.org dumped 2003-03-17 with terodump