From WIKIPEDIA, news reports and personal experience comes the following:

An EFP or explosively formed projectile/penetrator has a liner in the shape of a shallow dish. It was first extensively used in about 2002 and is much more destructive than conventional land mines or bombs. The force of the blast molds the copper, or other type of metal liner, into any of a number of configurations that can pierce even the most formidable armor, going easily through armored Humvees and even Abram tanks. EFPs can be expertly manufactured or crudely made. I.E.D’s or improvised explosive devices are less accurate but can be just as deadly.

An EFP eight inches in diameter can throw a seven-pound copper slug, becoming molten after discharge, at Mach 6, or a rough estimate of over 3,000 MPH or 2,000 meters per second. As a comparison, a .50-caliber bullet, among the most devastating projectiles on the battlefield, weighs less than two ounces but has a muzzle velocity of only 900 meters per second. (So… this projectile goes faster than the bullets coming out of a .50 caliber machine gun).

Some sources quote that over 75% of the U.S. fatalities in Iraq are from EFP’s. Despite fervent efforts to beef up armor in new ways, there have been no successes in opposing the effects of this weapon. Such devices create a greater fear for death in the average combat Soldier or Marine than any other device. They are constant reminder that adversity comes in many forms for both the military member and the families who suffer from the loss of their loved ones.