ClassBreaksRendererDefinition defines the parameters for drawing features based on the values of a quantitative attribute that is statistically grouped by a classification algorithm.
To set up a ClassBreaksRenderer you first need to know how many classes you will have. Then you specify a particular classification algorithm (e.g. natural breaks, quantile, equal interval) to your data to compute class breaks for your renderer. You also provide the color ramp. Once you define a renderer, you can call a FeatureLayer's CreateRenderer and SetRenderer methods to create and assign a renderer to a feature layer.

Reference

ClassBreaksRendererDefinition Members
ArcGIS.Desktop.Mapping Namespace