Symbolizing Data in ArcView's Legend Editor
Open a project in Arcview and double click on a theme in the legend to bring up the legend editor.

Under "legend type" you can choose one of the following types of symbolization:

Unique values map:
a different color is used to symbolize each value in an attribute. Three types are common.
Attributes that describe the name, type, condition, or category of a feature.
Attributes containing measurements or quantities that are already classified (0-5, 6-10).
Attributes that uniquely identify features.
examples include:
states by name, countries by type of government,

Graduated color maps:
have a series of symbols whose colors change according to the values of a particular attribute.
examples include:
temperature, property tax,

Graduated symbol maps:
Are similar to Graduated color maps, except the variation is in the size of a point or the width of a line symbol.
Most useful for showing rank or progression, such as the size of cities, or the traffic volume on roads.

Dot Density maps:
symbolizes polygon features using dots inside the polygon to represent an attribute value. Each dot represents a specific value. Warning: dot density is among the most misused ways of symbolizing data.

Classification methods
In the legend editor, click on the "classify" button in order to change the method used to create categories for that theme.

Natural Breaks:
This method identifies breakpoints by looking for groupings and patterns inherent in the data. Boundaries are set where there are relatively big jumps in the values.

Quantile:
each class is assigned the same number of features. (the first five, second five, third five, etc.)

Equal Area:
classifies polygon features by finding breaking points in the attribute values so that the total area of polygons in each class is approximately the same. (These are typically similar to quantile classes.)

Equal interval:
divides the range of attribute values into equal sized subranges.
For example, if temperature ranges from 0-100, with 5 categories --> 0-20, 21-40, 41-60, 61-80, 81-100.

Standard deviation:
shows you the extent to which an attribute's values differ from the mean of all the values. Arcview finds the mean value, and then places class breaks above and below the mean at intervals of either 1, .5, or .25 .

If you normalize an attribute, Arcview divides each of its values by another number to come up with ratio values. Find the legend editor's "normalize by" pop-up menu (below the classification field) to choose a way.
Two ways:
Normalize by "percent of total" enables you to communicate the relative size or importance of something. (% rather than volume).
Normalize by the values in another attribute. When normalizing data by another field, you can take into account the spatial variation in another phenomenon that might influence or control the attribute you are classifying the theme with.
e.g. sales values by state may be normalized by population to minimize the influence of varying population to give you a better idea of how sales are going in states with big and small populations. If you normalize the population by the area, you get a map showing population density.

Based on the ESRI manual "Using Arcview GIS" 1996 pp.98-111