The intensity is a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the earth's surface and on humans and their structures. Several scales exist, but the ones most commonly used in the United States are the Modified Mercalli scale and the Rossi-Forel scale. There are many intensities for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake.
See also: The Modified Mercalli Intensity (MMI) Scale
Map showing intensity for the New Madrid earthquake. (Image courtesy of the Central U.S. Earthquake Consortium)