We provide a dynamical portrait of singular-hyperbolic transitive
attractors of a flow on a 3-manifold. Our Main Theorem establishes
the existence of unstable manifolds for a subset of the attractor
which is visited infinitely many times by a residual subset. As a
consequence, we prove that the set of periodic orbits is dense, that
it is the closure of a unique homoclinic class of some periodic
orbit, and that there is an SRB-measure supported on the attractor.