Market segmentation involves dividing a large homogenous market into clearly identifiable segments based on similarities in customer needs, wants or demands. Effective segmentation allows companies to personalize marketing campaigns and reduce risk. There are five criteria for effective segmentation - segments must be accessible, differentiable, actionable, measurable and substantial. Segmentation bases include demographic factors like age, gender, income as well as behavioral and psychographic variables. While segmentation has advantages, it also has disadvantages like requiring extensive research and being an expensive process.