Original: Kasem is best known by name as a music historian and disc jockey, most notably as host of the weekly American Top 40 radio program from 1970 to 1988, and again from March 1998 until January 10, 2004, when Ryan Seacrest succeeded him.
Simplified: He was the host of the weekly American Top 40 radio show from 1970 to 1988, and again from March 1998 until January 10, 2004.