Systrace

Untrusted or complex applications can lead to security compromise.
Evil web pages may cause browser to send or delete files.
Bugs in an irc client can be used to install back doors.

Confinement or sand boxing to restrict the operations applications may perform.
Goldberg and Wagner's Janus. Drawbacks:
Policy difficult to define.
Applications may not chdir, etc.

Systrace offers fine-grained confinement for multiple applications with multiple policies.