Clusters are the dense inner regions of a wide-spread hierarchy of
young stellar structures. They often reveal a continuation of this
hierarchy inside of them, to smaller scales, when they are young,
but orbital mixing eventually erases these subparts and a only
smooth cluster or smooth unbound group remains. The stellar
hierarchy follows a similar structure in the interstellar gas,
which is presumably scale-free because of supersonic motions in
the presence of turbulence and self-gravity. The efficiency of
star formation increases automatically with density in a
hierarchical ISM, causing most dense stellar groups to be
initially bound for local conditions. In lower pressure
environments, the infant mortality rates should be higher. Also
following from hierarchical structure is the cluster mass
distribution function and perhaps also the cluster size
distribution function, although the predicted mass-size relation
is not observed. Cluster destruction is from a variety of causes.
The destruction time should depend on cluster mass, but the
various groups who have studied this dependence have gotten
significantly different results so far.