WIKIContextual advertising
Contextual advertising is the practice of selecting adverts based on the content of a web page. This increases the relevance and click-throughs for advertisers. The most prominent exponent is Google. It is not based on profiling of users or online profiling so stands in contrast to behavioural advertising.
Privacy concerns
Some people question the practice of delivering adverts against content based on email, for instance, on the basis that it could be seen as "reading" or "intercepting" email from third parties. On a legal definition of interception, this may be unlikely however.