400 Days is Telltale's first foray into DLC (not including the original The Walking Dead episodes themselves, which were marked as DLC out of necessity on XBLA). The 400 Days downloadable content is for The Walking Dead Season One, and is one episode containing five all new stories to play through. It is scheduled to be released this summer.
A bundle for PS Vita has also been announced. The bundle will include the original 5 episodes and 400 Days for the PlayStation Vita, and is scheduled to be released this August.
IGN has posted a trailer for the 400 Days DLC, which is embedded below: