Countdown (actual title according to the indicia of all issues) was a limited series (51 issues) that follows 52. The series took you to every corner of the DC Universe and Multiverse and acted as the spine of the DCU, with every major event and character spinning in and out of the story at some point. Like 52, Countdown was set to come out weekly, but unlike 52, each issue did not necessarily correspond to one week. Instead, every four issues cover a month of time. As the name suggests the issues of Countdown were be a countdown, going from 51 to 1.
Note: Indicias differ from the cover dates usually by 2 months (example: issue #1 is cover dated May 9, 2007, but the indicia indicates a July 2007 date).