An influencer is a person with a high number of followers on one or more Social Media channels, he/she is a thought leader or opinion leader. There are plenty of examples: The blogger (and Internet intellectual) Sascha Lobo, YouTuber Bianca Claßen (channel: "Bibis Beauty Palace"), YouTuber Rezo or the Instagrammer Hannes Becker.
The influencer and book author Marie Luise Ritter considers Influencer Marketing to be a mix of "recommendations from a friend" and "classic advertising". In my opinion, the often-used comparison between Influencer Marketing and Testimonial Advertising is a bit misleading, as influencers often have a professional expertise, from wich opinion leadership is derived. So it's not just a matter of popularity and reach, but also of expertise. It's different for testimonial advertising: When the ex-football star Oliver Kahn promotos the chocolate snack bar Lion in an advertisement, the promotional effect comes from his popularity as a football star, not from his expertise on chocolate snack bars.