The collision occurred at 16:56 UTC and destroyed both the 560kg Iridium 33 and the 960kg Kosmos-2251. The Iridium satellite was operational at the time of the collision. Kosmos-2251 had gone out of service in 1995. It had no propulsion system, and was no longer actively controlled.
Calculations made by CelesTrak had expected these two satellites to miss by 584 meters.
The satellites collided at a relative speed of roughly 35,000 km/h (22,000 miles per hour). This collision created over 2000 large space debris fragments that can be hazardous to other satellites
A small piece of Kosmos-2251 satellite debris safely passed by the International Space Station at 2:38 a.m. EDT, Saturday, March 24, 2012, at a distance of approximately 120 m (390 ft). As a precaution, ISS management had the six crew members on board the orbiting complex take refuge inside the two docked Soyuz rendezvous spacecraft until the debris had passed.
In 2016, Space News listed the collision as the second biggest fragmentation event in history, with Kosmos-2251 and Iridium 33 producing respectively 1,668 and 628 pieces of catalogued debris, of which 1,141 and 364 pieces of tracked debris remain in orbit as of January 2016
See also discussion at the time and links to various news articles:
Collision 500 Miles Above The Earth | Hacker News
U.S.-Russian Satellite Collision Sends Debris Flying | Hacker News