An exokernel eliminates the notion that an operating system should provide abstractions on which applications are built. Instead, it concentrates solely on securely multiplexing the raw hardware: from basic hardware primitives, application-level libraries and servers can directly implement traditional operating system abstractions, specialized for appropriateness and speed.
For examples of the ExoKernel design, look at the OpenDirectoryProject (dmoz.org) pages on the topic.
