A system is disclosed that maps user IDs (such as IP addresses) and geographic regions to particular categories or “segments” reflective of user profiles. The mappings may be based on behavioral data reflective of user browsing activities, offline data (e.g., survey data or demographic data), or a combination thereof. In one embodiment, the mappings are generated using data collected by a plurality of Internet Service Providers (ISPs) regarding their subscribers. The system also includes a query interface that enables content providers to retrieve segment data associated with particular user IDs.