Not Yet Released in the TCG or OCG refers to a card that doesn't exist in the format in question, but has a confirmed debut. It is sometimes possible to own a card before it is officially released (like from an English Duel Terminal). However, it is considered an illegal card until the release date.
Traditional/Advanced Format Lists & Statuses
Mega-Banned - Forbidden - Limited - Semi-Limited - Unlimited
Not Yet Released - Legal - Illegal
Status Unknown - Present - Not Present
All Lists
May '02
Sept '03
Apr '04 - Sept '04
Apr '05 - Sept '05
Apr '06 - Sept '06
Mar '07 - Sept '07
Mar '08 - May '08 - Sept '08
Mar '09 - Sept '09
Mar '10
Sep '10
Chart - World Championship 2008 Lists