It'll be in the form of new character costumes, new weapons and introduce new monsters. Some of it will be reserved as pre-order bonuses for retailers.
No downloadable content was planned for Final Fantasy XIII, although Square did at one point seem on the fence about it. Instead the studio decided to throw all their developer weight behind the full sequel title.
Final Fantasy XIII-2 releases on Xbox 360 and PS3 December 15th in Japan, January 31st in North America and February 3rd in Europe.