If you were looking for Cliff's father, also named 'Clifford C. Clavin', please see Cliff Clavin Sr
Clifford C. Clavin, Jr. is one of the main customers who attends Cheers.
Portrayed by John Ratzenberger, Cliff is a know-it-all mailman, who is Cheers' resident trivia expert. He is often seen sitting next to his best friend Norm Peterson at the bar. Most characters do not take Cliff very seriously, being the butt of most of the other characters' jokes; mostly due to still living with his mother and not having a sex life. Though we hear he's been intimate with two women--Maggie on "Cheers" and Sheila on "Frasier." Clavin is a recurring character during the debut season and a regular for all seasons after.
In the spin-off Frasier, he is only seen once during "Cheerful Goodbyes", where the regulars at Cheers throw him a retirement party. He originally plans on moving to Florida for his retirement, but then decided to stay in Boston (much to Carla's dismay).
However, in the Frasier episode "RDWRER" (pronounced Road Warrior), there is a middle aged man traveling the same route as Frasier in a Winnebago along with his mother. The man is named Clifford, likely as a reference to Clavin.