Hello Mohammad Zaman
CLNS is the OSI based service that provides connectionless network services, that is, connectivity between nodes. It essentially provides capabilities in an OSI network environment similar to those provided by IP and UDP together. UDP is mentioned here and not TCP because CLNS is strictly a best-effort service. CLNP is the actual protocol that is used to implement it.
ISO (the governing body that defines these standards) makes a distiction between the service offered to higher layers (CLNS) and the protocol used to implement it (CLNP). There is no equivalent structure in the TCP/IP world.
I hope this has been helpful!
Laz