foundation.  Some of the common reasons
are very large design loads, poor soil
conditions, or site constraints (like
property lines).

Piles are typically used in when bedrock is
non-existent (ie: along the beach) and
non-existent (ie: along the beach) and
between the piles and the ground.  They
provide bearing load based on friction are
driven into the ground.  Caissons efficacy,
by contrast, is predicated on a certain
embedment into "competant" material (ie:
bedrock).  Caissons are built by first drilling
(or hand-digging if they are shallow or if the
site is inaccessible) a shaft extending down
to hardpan or rock.  Next, re-bar cages are
constructed in the pits and finally they are
filled it with concrete.  Often adjacent
caissons are joined with grade beams (a
concrete "beam" resting at or slightly
below grad).  Subsequently, a wall or
building column is then built upon the
(subterranean) caisson.