Clean boost and a distortion circuit—like 2 effects in 1 box.
The Box of Rock is ZVex's first distortion pedal, highly specialized to simulate the "everything on 10" sound of a classic Marshall amplifier. You can use the Box of Rock effectively with many different amplifiers, but you might want to run it through a Marshall at least once. Use your guitar's volume control to adjust the amount of distortion you want, all the way down to very clean and clear with most drive settings.
Each of these distortion pedals is hand-painted, so each will be a little different from the rest. The ZVex Box of Rock also contains an extremely high-headroom, unity-to-50X gain booster with nominal input impedance and low hiss.
Note: Due to the nature of each hand-painted Box of Rock pedal, there may be a slight difference in design from the posted image.