Complex sentence: The SuperSonics (a.k.a. the Sonics) were a NBA franchise based in Seattle, Washington that played from 1967 until 2008, but relocated from Seattle to Oklahoma City, Oklahoma, and became known as the Thunder.
Split into simpler sentences: The SuperSonics (a.k.a. the Sonics) were a professional basketball franchise based in Seattle, Washington that was a member of the National Basketball Association (NBA). The Sonics played from 1967 until 2008, but relocated from Seattle to Oklahoma City, Oklahoma, and were renamed as the Thunder.