Behavioral Targeting refers to a range of technologies and techniques used by online website publishers and advertisers which allows them to increase the effectiveness of their campaigns by capturing data generated by website and landing page visitors. When it is done without the knowledge of users, it may be considered a breach of browser security and illegal by many countries' privacy, data protection and consumer protection laws.
When a consumer visits a web site, the pages they visit, the amount of time they view each page, the links they click on, the searches they make and the things that they interact with, allow sites to collect that data, and other factors, create a 'profile' that links to that visitor's web browser. As a result, site publishers can use this data to create defined audience segments based upon visitors that have similar profiles. When visitors return to a specific site or a network of sites using the same web browser, those profiles can be used to allow advertisers to position their online ads in front of those visitors who exhibit a greater level of interest and intent for the products and services being offered. On the theory that properly targeted ads will fetch more consumer interest, the publisher (or seller) can charge a premium for these ads over random advertising or ads based on the context of a site.
Behavioral marketing can be used on its own or in conjunction with other forms of targeting based on factors like geography, demographics or contextual web page content. It's worth noting that many practitioners also refer to this process as "Audience Targeting".
Read more about Behavioral Targeting: Onsite Behavioral Targeting, Network Behavioral Targeting, Behavioral Targeting Tools, Privacy and Security Concerns, Case Law