Symbology is the application of graphic symbols—varied by color, size, and other properties—to represent geographic features on a map. In ArcGIS Desktop, symbology is set on the Symbology tab of the Layer Properties dialog box.
Under the Features heading, there is one symbology method, Single symbol, which is applied by default when a layer is added to a map. In this example, all US states are drawn in the same color.
Under the Categories heading are methods for symbolizing descriptive data. In this example, the Unique Values method assigns varying line widths to a Roads layer according to whether the road features are freeways, highways, or streets.
Under the Quantities heading are methods for symbolizing numeric data. Here, census tracts are symbolized by population density. Low-density tracts are yellow, medium-density are orange, and high-density are brown.
The Charts heading contains methods for comparing numeric attributes by chart symbols. In this example, pie charts compare the percentage of owner-occupied homes (red), renter-occupied homes (blue), and unoccupied homes (yellow) for Southern California cities.
The Multiple Attributes heading has a method for symbolizing descriptive and numeric attributes at the same time. Here, the descriptive attribute is parcel vacancy. Vacant parcels are orange; developed parcels are pink. The numeric attribute is assessed land value. A small orange dot in a vacant parcel means the assessed value is relatively low; a large dot means the assessed value is high.