  The origin of Gamma-Ray Bursts is one of the most interesting puzzles in
recent astronomy. During the last decade a consensus formed that long GRBs
(LGRBs) arise from the collapse of massive stars and that short GRBs (SGRBs)
have a different origin, most likely neutron star mergers. A key ingredient of
the Collapsar model that explains how the collapse of massive stars produces a
GRB is the emergence of a relativistic jet that penetrates the stellar
envelope. The condition that the emerging jet penetrates the envelope poses
strong constraints on the system. Using these constraints we show that: (i) Low
luminosity GRBs (llGRBs), a sub population of GRBs with a very low luminosities
(and other peculiar properties: single peaked, smooth and soft) cannot be
formed by Collapsars. llGRBs must have a different origin (most likely a shock
breakout). (ii) On the other hand regular LGRBs must be formed by Collapsars.
(iii) While for BATSE the dividing duration between Collapsars and
non-Collapsar is indeed at $\sim 2$ sec, the dividing duration is different for
other GRBs detectors. In particular most Swift bursts longer than 0.8 sec are
of a Collapsar origin. This last results requires a revision of many
conclusions concerning the origin of Swift SGRBs which were based on the
commonly used 2 sec limit.
