LXC is the Linux Containers project, which is an application container approach that has been included in the official Linux kernel sources since 2.6.29. LXC userspace tools packages exist in many current (October 2009) Linux distributions, including Debian, Fedora and Ubuntu.
To quote from the LXC Cookbook by Kathy Staples:
Containers provide light vitualization at the OS level with subsystem isolation, each subsystem having its own namespace. This provides the ability to have a whole view of the real system as well as being able to assemble bits and pieces to optimize resource usage, security and provide mobility. Containers are a generic mechanism for saving the state of an application at any point, with the ability to later restart that application exactly where it left off. This is commonly referred to as checkpoint/restart (C/R). This can also be done dynamically from one machine and then moved to another.