Android is a multi-process system, in which each application (and parts of the
system) runs in its own process. Most security between applications and
the system is enforced at the process level through standard Linux facilities,
such as user and group IDs that are assigned to applications.
Additional finer-grained security features are provided
through a "permission" mechanism that enforces restrictions on the specific
operations that a particular process can perform, and per-URI permissions
for granting ad-hoc access to specific pieces of data.