The concept of Areas was introduced in OSPF or link state protocols in general and routers had different roles based on their location in the topology. We have backbone routers, Area border routers, ASBRs and so on. IS-IS is implementing the same concept of areas named as routing domains and introduced the following routers roles:
Level1 router is non-backbone router; builds only adjacency with neighbors in the same area. Level1 routers know only about intra-area routes or topology information and all they know about other areas (inter-area) is a default route pointing to the closest L1/L2 routers (similar to ABR in OSPF).
Level2 router is a backbone router and may build adjacency with any direct connected L2 router even if not in the same area. L2 routers are responsible for all inter-area routing and have all the information about the topology.
This router is similar to the ABR router in OSPF, it builds adjacency with L1 routers in the same area and L2 adjacency with other backbone routers. L1/L2 router functions as an area gateway for all L1 routers in the area. All inter-area traffic is sent by L1 routers to L1/L2 routers to traverse the backbone finding its way to other areas.
In the next post we are going to cover all of these topics using a configuration example; hope it clears some of the confusion about IS-IS.