Complex sentence: In Usenet slang, Eternal September (or the September that never ended) began in September 1993, the month that internet service provider America Online began offering Usenet access to its tens of thousands, and later millions, of users.
Split into simpler sentences: In Usenet slang, Eternal September (or the September that never ended) began in September 1993. In that month, the online service America Online began offering Usenet access to its tens of thousands, and later millions, of users.