The observed increase in star formation efficiency with average
cloud density, from several percent in whole giant molecular clouds
to ~30 percent or more in cluster-forming cores, can be understood as
the result of hierarchical cloud structure if there is a
characteristic density as which individual stars become well
defined. Also in this case, the efficiency of star formation
increases with the dispersion of the density probability
distribution function (pdf). Models with log-normal pdf's illustrate
these effects. The difference between star formation in bound
clusters and star formation in loose groupings is attributed to a
difference in cloud pressure, with higher pressures forming more
tightly bound clusters. This correlation accounts for the observed
increase in clustering fraction with star formation rate and with
the observation of Scaled OB Associations in low pressure
environments. "Faint fuzzie" star clusters, which are bound but
have low densities, can form in regions with high Mach numbers and
low background tidal forces. The proposal by Burkert, Brodie and
Larsen (2005) that faint fuzzies form at large radii in galactic
collisional rings, satisfies these constraints.