The holy trinity of media effects research is "agenda setting - priming - framing." We've used all of these terms at some point in this blog. Since they are central to all kinds of communication work - and policy work, to quite some extent - we'll introduce all three a little more thoroughly, starting with agenda setting.
Agenda setting means the ability of the mass media to bring issues to the attention of the public and, related, of politicians. The basic claim is that as the media devote more attention to an issue, the public perceives the issue as important. When the media take up a specific topic - such as climate change, or manager bonuses - they make us think about it. The theory was introduced in 1972 by Maxwell McCombs and Donald Shaw in their seminal study of the role of the media in the 1968 Presidential campaign in the US ("The Agenda-Setting Function of Mass Media").