They allegedly formed an agreement not to poach executives from each other unbeknownst to employees; the companies policed the pact and called out the others when there were breaches.
Steve Jobs seemed to be the ringleader in the alleged pact. A number of aggressive emails he sent have been published in which he threatens other HR departments that pursued Apple's talent.
Google's Eric Schmidt was also caught sending a damning email about the alleged pact that read, "I don't want to create a paper trail over which we can be sued later…"