The "Extremely Reliable Operating System", an experimental computer OS designed by Jonathan Shapiro (and a team of undergrads
) at the University of Pennsylvania
The intent of the EROS project is to create an OS that is both extremely secure and extremely reliable i.e. failure-resistant.
The security aspect of EROS is done by creating an object-oriented 'capability' system in which all system components (programs, files, users) are given capabilities to perform certain actions (read, write, delete, move) on other objects. This is more fine-grained than the UNIX system of permission-based access.
The reliability aspect includes a highly efficient snapshot system in which the state of the machine is maintained on the filesystem in a way that can be easily extrapolated. The idea being that if the machine running EROS loses power, it will resume its operation from the state it was in at power loss, something analagous to a laptop's 'hibernation' feature.
More detailed (and accurate) information is available at www.eros-os.org.