Thanks to Agents of S.H.I.E.L.D., we know that centuries ago, Asgard sent an army to Earth to fight in an unknown war. After the war was won, one of those soldiers decided to stay on Earth. He left behind his life of war and shattered the Berserker Staff
into three pieces, hiding it away until it was eventually rediscovered centuries later.
The Staff has the ability to grant its wielder superhuman strength, along with an overwhelming sense of rage and hatred. The strength, along with the rage, tend to fade fairly quickly once the wielder has put the Staff down, but it can have long-term mental effects on humans due to being forced to confront all of their pent-up anger.
Basically, the staff turns the person wielding it into a declawed Wolverine.