Stars form not in isolation, but as members of populous groups.  These
groups fall into three distinct types: T associations, open clusters,
and OB associations. The three have very different morphologies and
evolutionary paths. I suggest that these differences mainly reflect
the mass of the parent molecular cloud forming each entity. The
catastrophic fate of a massive cloud is illustrated through a
simplified model of the Orion Nebula Cluster. Much lower-mass clouds
form T associations, while those in an intermediate range spawn open
clusters. For the latter case, I show progress in understanding the
history of the Pleiades. This general model of group origins should
prove useful for the broad, cosmological issue of star formation
efficiency