PostEaseThe short answer
Influencer marketing is a brand partnering with a creator to promote a product or service to that creator's existing audience, in exchange for payment, free product, or another form of compensation. What separates it from traditional advertising is that the message is delivered through a person the audience already follows and, ideally, already trusts.
The trust being borrowed is the entire value
A brand pursuing an influencer partnership is paying to borrow the trust a creator has already built with a real audience. That is also why an undisclosed or clearly insincere partnership damages the trust, and with it the entire premise of why the partnership had value.
It spans a wide range of scale
Influencer marketing covers everything from a large creator with millions of followers to a "micro-influencer" with a small, highly engaged niche audience. A smaller, more relevant audience can often deliver more value to a brand than a much larger, less engaged one.