CCIE Routing & Switching V5:ISIS Introduction- Levels Types

IS -Intermediate System(the OSI term for a router)
IS-IS : Intermediate System to Intermediate System routing exchange protocol (the ISO protocol for routing within a single
routing domain)
IS-IS was originally designed for use as a dynamic routing protocol for the ISO Connectionless Network Protocol (CLNP);
Adapted for routing IP in addition to CLNP as integrated or dual IS-IS
IS-IS is a Link State Protocol similar to the Open Shortest Path First (OSPF)