FreeBSD Jails (O'ReillyNet)
Those familiar with Java recognize the security concept of a sandbox. For those that aren't, it's the concept that everyone gets a unique, well-equipped sandbox to play in, and a person in one sandbox isn't allowed into anyone else's sandbox, not even to share anything with anyone else. On FreeBSD, jails implement this concept -- they keep processes in their own part of the system, denying access to anything else."