A hashtag campaign is a marketing initiative used on social media to gain attention and engagement for a topic, product, or idea. Effective hashtag campaigns involve researching brands and competitors, defining goals, creating resonant hashtags, executing with influencers, boosting the campaign, and measuring growth. Companies provide examples like Visa's #TeamVisa and Dove's #ShowUs project, which aims to showcase a more inclusive vision of beauty by featuring real, unaltered photos shared on social media.