Third party data is commonly data related to a visitor or user and which has been previously anonymously aggregated from multiple websites publishers or marketing service vendors. The data is segmented into targetable audience clusters or available at more granular levels.
Third party data can also originates from offline sources.
Third party data is used by advertisers / agencies for advertising purposes, by merchants for personalizing offers and contents and by publishers for valuing their ad inventory. Third party data is also used in ad exchange operations.
Third party data is offered by ad networks or data vendors through DMPs and rises privacy and trust issues as it is not always clear and transparent how the data is collected.
Third-party data is commonly "opposed" to first-party data.
An illustration of how digital actors trust third party data according to an Exelate study: