Countdown starts at # 51 and then each week a new issue comes out it will "countdown" to #0 where the next big event will happen. But Countdown itself is considered an event. Promo art such as...

Suggests that the great disaster will occur DURING Countdown and that it will involve Darkseid.
The Great Disaster is an event that rids the world of all superheros and leaves just one human boy alive: "Kamandi". Kamandi was named after the bunker he's found in after The Great Disaster, Bunker Command D. During DC's "The Battle of Bludhaven" which occured during the ending of Infinate Crises, Bunker Command D is clearly seen.
Kamandi himself has also been popping up all the sudden in other titles since a long absense. During one reality hopping storyline in Superman/Batman, Superman and Batman find themselves saving Kamandi who acts like he knows Superman and then begs him to stay because "the world still needs superheros".
The Great Disaster is just one of the events of Countdown though, like 52 it'll have multiple occuring story archs.
Including:
"Seduction of the innocent" (featuring Eclipso and Mary Marvel)
The return of Barry Allen
The introduction of Kingdom Come's Red Robin
"The search for Ray Palmer"
A heavy focus on the characters Jimmy Olson, Mary Marvel, Pied Piper, and the Trickster.
Also, when asked what plot point would suprise it's readers the most, writers of Countdown said:
"Sue Dibny".