Agenda-setting theory explains the media's power to influence public perceptions of what issues are important, originally developed by McCombs and Shaw during the 1968 U.S. presidential election. The theory highlights various applications, including political campaigns and commercial branding, asserting that media coverage shapes public awareness and concern for specific topics. Key concepts include public agenda, media agenda, and the distinction between framing and priming, while critiques address the complexity and passive audience assumptions associated with the theory.