CASS stands for "Audio Cassette" which is the standard tape we've all known and loved for so long. Cassettes became popular in the early 70's but declined during the 80's as the CDA gained ground. Compact discs have all but ended the use of audio cassettes. We expect cassettes will vanish (like "reel to reel" & "eight track" tapes) by the year 2007.
In most cases, purchasing MP3 discs rather than cassettes or CDA discs will save you enough money to buy any MP3 player you want for home or car use.