Following first stage separation, the Falcon 9 used a series of engine burns to adjust its course for landing. While the rocket made it to the landing pad, the grid fins used to control its descent ran out of hydraulic fluid. The stage lost control and crashed. All in all, it was an impressive feat, and SpaceX intends to try again until they stick the landing.
SpaceX released this Vine video of the landing this morning:
Late last night, Musk tweeted four images of the rocket impacting the landing pad, taken by a camera aboard the drone ship. Here is a video combining all four frames: