If you need to drive a screw into concrete, you've got two choices. Either you use a large rotary hammer or a drill driver with a hammer-drill function. Both tools pound the bit while it spins, pulverizing concrete, but the two differ in the mechanisms that do the actual pounding. In a rotary hammer, a cylinder of air is compressed by a piston, which in turn beats the bit. In a hammer drill, two ribbed metal discs click in and out against one another, causing impact. Among tradesmen working daily, rotary hammers are preferred because of their superior strength and shock-absorbing qualities. For the occasional user, the large, bulky tools are often overkill—a powerful all-purpose drill driver with a hammer-drill setting makes more sense.